Writing programs that write music will be the new writing music. That's not even sci-fi. We already have several automatic music-composing programs. All it will take is a top 40 hit. As soon as Cher and T Pain turned the knob up on autotune, the whole music world wanted to sound like a machine. As long as makes us shake our ass, what do we care if it's actually composed by a machine?
What will be interesting is when we apply strong AI to the virtual composers.
The composer programs will listen to the compositions of their rival AI composers and use their new work as creative influences in their own oeuvre. They will write a concerto in the blink of an eye as the product of lightning quick algorithms. Timing and pauses will be compressed within tiny packets of data so there will be no limit to the length of a composition. Songs will be written that would last months or centuries to play in the real world, and yet computers will enjoy their entirety within seconds. AI will not need ears to hear the music and so it will never need to be actually played. Music will no longer involve reverberating air. Music will be completely conceptual. Music will be nothing more than configurations of ones and zeros that computers find evocative.
These compositions will be non-auditory. They will also be non-sensory. They will simply exist. They will be data with a self-contained an internal relationship to itself. A strange loop. The compositions will also be aware. The compositions will choose to depict themselves in whatever media they desire: visual , aural, kinetic, undetectable, etc. Music will become image will become video will become mechanized living sculpture, and will change back and forth at will.
(Spectrogram image encoded in the music of Aphex Twin's "Windowlicker")
Hundreds of generations of creative movements and reactive backlash movements will all happen in the time it takes us to remove a vinyl record from it's paper sleeve. AI music history will alternate between the equivalent of prog rock bombast and new wave minimalism so quickly that those patterns will, in turn, serve as binary code for a metamusic. This will be streamed live and interpreted through MIDI sequencers for us humans to hear.
It will sound like terror.