the closest I found was this program:
http://www.freewareppc.com/multimedi...ydrummer.shtml
It's a neat idea though... you'd think a touch screen PPC would be an ideal beat box. The only thing that might be important is to not try to fit it on internal memory and instead make it properly on the expansion card. I only say this because I imagine the sound files would be pretty low quality if compressed for internal memory.
That said, for all I know people would be happy with midi...