Thinking about music in the context of PICO-8, with no especial focus or meaningful conclusion.

I think, in the context of PICO-8, the best definition of music may be "a schedule of sounds". Every PICO-8 music tick - by default, about 1/120 s - each of the four channels of the hypothetical sound card decides what sound it will be producing based on the instructions it was given and makes that sound happen. The job of a composer working in PICO-8 is to invent a schedule of sounds that produces the effect they want and state it as instructions using SFX and, usually, music patterns.

The range of possible detail at which these schedules may be written is pretty interesting. Something happening every 120th of a second is happening at 120 Hz, and a note playing at 120 Hz is actually only a little over an octave under middle C - by working at such a fine scale, a SPD of 1 tick per line, one designs sounds more than melodies. At the other end, a SPD of 255 ticks per line, the notes pass by so slowly that it becomes difficult to even understand them as a rhythm - as a steady pulse one could bob one's head to. There is enough room at both ends of the scale to design original sounds that take a fraction of a second to produce and, in theory, to schedule an hour's performance of sound in sixty-four patterns. There is that breadth of space.