Somewhere in between. It was a static composition (as in, it had a start and an end) but there were multiple loops per instrument, so more competent players would reach a more frenetic musical performance of the piece if they kept more of the balls in play.

The audio engine (the bit I worked on) was in effect a stem based mixer, but with a state transition diagram per stem, with multiple loops available. Depending on the route through the transitions (triggered by events from the game play, for example, how many balls the player is keeping in play) it was possible to reach more complex or simpler performances of the same musical piece, so the players decisions and ability would affect how the performance of the music, not the music itself if that makes sense?