Vel and I tried to do this very thing with the freeware version of Shivah. During the final fight scene, Vel had this neat idea to add extra layers to a music track as the hero got closer to victory, and to remove them as he got further away. Unfortunately, I could never get it to work. The tracks would never synch up properly, and even cross fading sounded weird and wrong. So we scrapped the whole idea. A shame, really, since Vel put all that work in.