Decompose the music into waveforms for each instrument. Then, for each instrument find common passages or sequences of notes. Substitute one representative version for all of the similar sequences.
This is a highly lossy scheme, but if feasible might be a passable scheme. The sound quality isn't degraded at all... Not for serious uses, of course, but it would be fun to see how much a song can compress.
Pop music and techno might compress down to a few kilobytes!