Never tested this..but I suppose that if you changed the music files within "Music.pk2" with your own (making sure the file sizes are the same, will be very hard to do.[maybe they do not need to be the same size >.> I have no idea]) then the music in game should change.
Edit: I just tried opening up a song with a hex editor. It works

You can add null bytes to songs to match file sizes..so..find a song that isn't as big as the original file (or the same) and add to it? You can try if you like, no garuntees tho.
----You still have to extract Music.pk2 and then inject it back in =o