We should talk, perhaps via PM, about what kind of music you're looking for. One of my ongoing projects -- between working on levels -- is just finding & downloading both copylefted (i.e. can be used with just a CC-BY-SA & the original creator's name/s so long as the use is not-for-profit) or public domain music and copylefted or public domain sound effects for ambience use, stuff that I think would work well with KS even if I don't have a particular level in mind for it. If I had an idea of what you were looking for, either by description or from examples, I could keep an ear out for you. I have the one song you added for Falling Water part 1, of course, but I don't know how representative that is of what you're looking for in general...
Also, I have a mic, Audacity, and fair facility at both voice acting and foley work, so if all else fails I could make at least some effects for you.
That sounds rather awesome. I've been thinking about sound a lot recently; in fact, I'm leaning towards learning to make my own music, at the moment, because I seem to have a fairly precise idea of what I want, and it's not something that's represented by any music I've been able to find so far. (No, the song from part 1 is not representative. Although I do like it.)
But, I wouldn't be against a collaboration, either. I can't really say for sure, at the moment, though, because I feel my ideas about the sound of FW are still far from final. I think I need to mull this over some more. But, if you stick around a bit, we could talk about this later, when my thoughts have progressed a bit. We seem eerily compatible in other areas, so maybe we'd be compatible in the sound & music area, too. *g*