So, I designed and built this back in October of 2013. As is, this is basically a tone and light sequencer. It uses piezos for audio input and output wired through a programmable microcontroller (i.e. an Arduino Uno). I've since programmed and mounted it on a piece of marble tile relating velocity to pitch, more like an actual musical instrument.
Cost could be improved using a more general ATmega328 chip. Sensing could be improved by mounting the piezo on galvanized steel, like an electronic drum. Eventually, I'd like to put some minor pentatonic scale through a transformer -- up the output for a real sound and light show. (If I can get my cat to stop eating the wires!)
It counts impacts, too.
This was filmed on a HTC Incredible. It was enhanced using YouTube Enhancement (Fill Light +1, Image Stabilize).