In the last 15 years there has been a major change in how popular music is created and therefore how it sounds. I believe that the core of this change is that the traditional connection between time and music has been radically altered. This change has been driven by the adoption of the digital audio workstation (DAW) as the default platform for music production.
To illustrate, let's consider the iTunes top 100. The vast majority of songs on this list were never "performed" by musicians in the way that we usually think of a performance. They were cut and pasted together in the computer. Even genres like Country or Metal, which have strong traditions of musicianship, are now created in this way.
As a further example of how different music production has become, many of the "sounds" that you hear in contemporary pop music are not only cut and pasted performances. Some are actually never sounds until the moment you hear them! They were created by "virtual instruments" inside the computer, which generate only data, not sound.
As someone who learned music production in the pre DAW era, then made the switch to working exclusively on a DAW, I must admit that the change in how music sounds has been more profound than I ever could have imagined. The trend has really accelerated in the past few years. Though the switch to DAW based production was largely complete by about 2002 or so, the cultural concept of what music is has lagged behind. We are now fully in the era of DAW based music. There is now a whole generation of producers who have only used a DAW for production. The changes in both the perception of music and the objective qualities is permanent. The operative cliches to describe this shift would be: we've crossed the Rubicon,opened Pandora's Box, and the horses have left the barn!
The DAW has affected many aspects of music, but I think the fundamental change is the ancient relationship between time and music. At the simplest level, music is a sequence of sounds played one after another. The relationship of those sounds to each other in time is what creates melody and rhythm. A simple illustration of how tightly time and music are connected is that we generally think about a piece of music as having a beginning, middle, and end. This corresponds to our perception of time as linear: the beginning is always first, then the middle, then the end.
The specific aspect of DAW technology that is driving this change in music/time relationship what is called "non-linear" or "random access" editing. The radical technology of the DAW is the ability to cut and paste pieces of "sound" (really data representing sound) and to jump instantly from any part of a song to another. It is trivial to take a sound from the end of your song and put it at the beginning. Routinely choruses are composed and performed only once by a singer, then are pasted at every chorus.
Many musicians who grew up in the DAW era would be surprised to find out that not too long ago you actually had to sing a chorus, along with all of the harmonies, every single time that you wanted to use it in a song! While the technology to use cut and paste techniques on vocal recordings has existed for quite some time (since the beginning of tape recording), until the arrival of the DAW it was usually more trouble to do this than to just perform it multiple times.
Another prime example of how the DAW can of our usual sense of time and music is the "mashup". A mashup overlays performances that were not originally played or intended to be listened to together. Two or more pieces of music created at different times, perhaps decades apart, are combined to create an artifact that is far removed from any historical conception of a musical performance. The concept of linear time is almost totally subverted (along with a bunch of other musical conventions).
The general stylistic trend in modern pop music production is the use of "loop" based composition techniques. A loop is any sound that repeats. It can be a drum, a sample from a record, a sequenced piece of electronic music, a vocal sample, anything really.
Once you have your loop (or "sample") it is trivial to repeat a loop for any amount of time in the computer. The samples that are looped to create the foundation of the song are colloquially referred to as a "beat". Note that the term "beat" does not just reference a drum loop. It is the whole collection of sounds that are looped. It can include drums, traditional instruments, samples from other recordings, voices, sounds created by virtual instruments, or found sounds.
The looped beat is emblematic of the broader change in the time/music relationship. A beat does have a beginning, middle, and end (it's essentially a very short piece of music). But, by repeating it over and over the linear nature of the loop itself is obscured. Perceptually the beginnings and ends of the loop blur into a continuous stream of sound. On the larger timeline of a piece, the loop will sound the same no matter where you are in the song, which weakens the sense that there is a "progression" that happens over the course of the composition.
The stylistic precursors to the type of music that rules the charts today are found in early experiments with tape loops, the use of turntables to manipulate and "scratch" vinyl albums by DJs, and MIDI sequencers.
The very first loops in music were literally loops of magnetic audio tape that were jury rigged to play continuously in a tape recorder. The process was touchy and awkward. Tape loops were the foundation of some huge hit songs in the 60s and 70s like The Bee Gees "Stayin' Alive" and The Beatles "Tomorrow Never Knows." But they remained a bit of a novelty and drum beats were primarily performed by human drummers until the advent of widely available MIDI sequencers and drum machines.
The use of MIDI sequencers, drums machines, and synthesizers in pop music began in earnest in the 1980s. They were heavily used in electronic dance music, as well as other pop styles, including rock. This type of music was generally constructed out of "sequences" of musical data that would trigger the sounds. The "sequencer" was either a dedicated piece of hardware or a computer program that could edit MIDI data.
Sequencers naturally lead to music based on loops and cut and paste techniques. Even so, most popular music in the 80s retained traditional conventions like having a verse and chorus. It was also common to record more traditional instruments like bass, guitars, drums, keyboards in combination with sequenced sound. In addition, the recording process was still very much dominated by both linear tape formats. Vocals might not be sung all the way through a song, but at some point each vocal that you heard on an arrangement was actually sung. It was also tedious to restructure an arrangement once it had been committed to tape. This acted as a limit on the adoption of the kind of production styles that are popular now.
The apotheosis loop based production that predates the DAW occurred in the 90s with the advent of the dedicated music worksttions like the Akai MPC 2000. The MPC is a combined drum machine, sampler, and MIDI sequencer. The whole "beat" for a song could be created and arranged inside a single device. The MPC 2000 was used to create many of the definitive rap songs of the 90s by artists like Wu Tang Clan and Dr. Dre. It remains a tool of choice for many current producers. (Here's a link to an interesting article on Kanye West's production process using an MPC along with other samplers and keyboards.)
Parallel to the innovations in MIDI sequencing, Hip-Hop DJs used the quasi random access nature of vinyl LPs to access time shifting techniques that they incorporated into what are essentially more traditional linear performances. The first of these was crossfading from one track to another, which removes the beginnings and ends of songs to stitch together an extended segment of music. (At good club DJ can give dancers a subjective sense of timelessness or suspended time through this approach.)
Record scratching was a another time shifting innovation that Hip-Hop DJs (turntablists) pioneered. By moving a segment of music on a vinyl LP back and forth under the needle (meaning a bit of the song plays forward, then backward etc) what was originally intended to be a segment of sound heard as part of a linear sequence is re-purposed into a new instrument. Both of these techniques (crossfading and scratching) rely on the ability to pick up the stylus and move it quickly to another section of music as a kind of quasi "random-access" that subverts the original intention of how LPs were meant to be heard (from beginning to end)
It was in the Hip-Hop genre that what I consider the first true loop based compositions to reach a large audience arose. It is a clear case of how a specific technology influences the development of a musical genre. Hip-Hop artists began constructing songs that were anchored around a repeating drum machine beat. With the advent of affordable digital samplers, they began to also incorporate "samples" from other pieces of music. A MIDI sequence was used to trigger the playback of the samples. This sequence was then easily "looped" for the duration of the song. Rappers would perform vocals over this "beat" to introduce dynamics and rhythmic variation to the song. A turntablist was a key part of the composition, who would scratch and lay samples in real time over the top of the beat. The Bomb Squad was the production team for Public Enemy and they used these techniques to create some of the definitive rap songs of the 80s and 90s.
Since it was hard to create more complex sequences with different sections in these hardware sequencers, a lot of songs tended to have one main section that repeated with no alternate parts. Over time, what was first a matter of convenience, using a single loop as the basis for a whole song, evolved into an actual stylistic convention that remains a foundational element of various "Urban" genres of music to this day.
While the early use of tape loops, MIDI sequencing and turntable techniques foreshadowed what was to happen to music with the rise of the DAW, the technical difficulty of executing the time shifting techniques and the essentially linear nature of the mediums kept these stylistic innovations from dominating the pop landscape until recently. Before the use of the DAW in production all recording media was linear (tape). So the time shifting techniques of these early technologies coexisted with more traditional linear approaches to music.
A beat, or a shorter sequence of sound, was either recorded onto tape or synced to play along. From that point, the sounds had to be laid onto the tape in a more traditional manner. This might include performances of musical instruments, vocals, or the recording of samples onto the tape in specific locations. It was a hybrid approach. The resulting music, at it's best, had a sort of organic quality with mechanical sound elements.
Working on a recording always had a stage where a tape deck was involved. It had to be constantly rewound and forwarded to locate parts of a song to work on. The rhythm of this operation of the tape deck transport (play, fast forward, rewind, record) was a constant anchor to the linear nature of the medium for the producers, engineers, and musicians whe were crafting a song.
Contemporary pop music is now produced almost exclusively in a DAW using loop based and cut and paste techniques. This has leads to some distinctive qualities that present unique challenges and opportunities for music creators. The most obvious characteristic of music produced in a DAW is that it so easy to loop and repeat sound that the resulting music can feel static and monotonous. Introducing the variation necessary to create a sense of "movement" requires specific attention. This shows an interestng contrast with the previous era of linear tape based recording. Large sums of money were spent on pop productions to get parts of an arrangemnt to be consistent and vary as little as possible over the course of a song. In short, getting a chorus to sound the same everytime it happened in a song was difficult and time consuming. Now it is triviallly easy. In contrast, now the problem for producers is to introduce enough variation into a production to make the arrangement engaging to a listener!
One of the main differences between loops in a DAW compared to the loop based production approaches I discussed earlier is that they are exact, bit for bit repeats of the sound. This was not practically possible using a MIDI sequencer. Even all in one workstations like the AKAI MPC had an imprecision in their timing, and so the sound of each iteration of the loop was not exactly the same as its predecessors. Even if it's on a subconscious level, the effect of an exact repeating loop on a listener is different than repeating sounds that have some variation.
Creative people tend to rise to challenges,and the best producers of our era have been resourceful in crafting songs that transcend the limitations of their medium. A song that I think embraces this new medium and transcends its limitations the recent, super duper smash hit Somebody That I Used To Know by Goyte (that's a spotify link). I think the song is quite stunning. It has a very effective use of traditional sounds, pop hooks, but is produced in a thoroughly modern DAW based style.
Like all musical technologies and mediums, the DAW has changed the sound of music. However I think what differentiates it from earlier generations of technological change is that is disconnected musical forms from linear time. It is a singular point in musical history, and the change is so profound that it can't be undone.
Some musicians will continue perform music live and in realtime, just as it always has been. But we are already seeing the breakdown of traditional concepts of live musical performance with the increasing use of pre-recorded music in concerts, DJ "performances," and even more jarring time based disruptions, like the recent resurrection of Tupac Shakur as a performing hologram at the Coachella Music Festival.
I predict that the older, traditional concepts of musical performance and recordings as anchored in time will be continue to be practiced for a long time. But it will be either in the spirit of maintaining musical traditions, or as conceptual tools enlisted to break free of the new orthodoxy of DAW based music. What is old now, shall be new again, but never the same!