This projects aims to create a MIDI-Shotgun (Mitra, in italian). When the button is pressed Arduino sends MIDI (http://en.wikipedia.org/wiki/MIDI) notes to the soundcard/syth/whatever-you-connect: if the distance from the ultrasonic sensor changes also the note changes, with the potentiometer you can alter the output frequency.
There is also a led... but just for fun :)
The result is something really really insane, noisy, funny and disturbing
You should build your own serial-to-midi adapter in this way:
Fritzing schema:
Demo:
Based on: http://minucci.net/blog/653