This is an interesting musical interface called the PianoDuino. It uses the SoundCipher library for Processing and an Arduino to create interactive sound. Check out the web site for a little more information, and all the code needed to build your own.
PianoDuino is a simple experiment that integrates Arduino, Processing and a library to manipulate sounds, the SoundCipher. The idea was to try the Multiplexer / demultiplex 4051.
In the Maker Shed: Make: Arduino