Back in the days when I used Windows
I had a free/shareware program called Soundscape
that does this. You fiddle with the knobs - very easy to use - and it
plays. And the music
Some guys I know at our university are doing this, too, but as a multiuser project: anyone can change the settings over the Internet.
I once wrote a simple music generation program in Hypercard.
I've also patched Rosegarden to support musical transformations. A transformation is simply another piece of music, which is interpreted as a generic transformation, to be applied to the whole piece, according to generic rules. Yes, this makes sense.
I never specified the exact rules, though - it was still under development.