Over at NerdKits, Humberto shows how to build a piezoelectric-based sound meter fed to an LED display, using their USB NerdKit. By using the piezoelectric buzzer that comes in the kit as a microphone, and adding a transistor and a few other components, you can create a sound meter and send its output to an LED array, an LCD screen, or other display. They used an LED display version at the recent MIT Battle of the Bands to graphically display the sound volume in the room.

Piezoelectric Sound Meter