In fact, the only real constraint is the scope of our imagination. Sound effects and sounds. Any sophisticated midi musician will appreciate the difference between these two creatures. Sound-effects are fun to play with, electronically. General Midi offers us presets of a tweeting bird, gunshot, applause, a helicopter, a telephone ring, etc.
Musical sounds are something else.
First, and foremost, to be so qualified, they must be music . New musical sounds. On a keyboard, or in a midi arrangement, we could now combine the sounds of instruments, like merging piano and violin tones. And dozens of other maneuvers. Admittedly there is something hypnotic in the possibility of inventing novel musical sounds.