Liquid and solid crap don't have much to do with music, but with gas phase, just maybe, you could train your cat to fart something simple like "Taps". "Kitten on the keys" would perhaps be expecting a little too much.

I've never measured the density of cat farts, so don't know what the velocity of sound and resulting frequencies would be. You may even invent a new key that way. Cows fart methane, and they might be good for a harmony part for the tune.