Checkout this compact LED light bar controller
created by Adam Greig!
Sound is picked up by an electret mic, amplified with an opamp, and then ADC'd to an ATtiny microcontroller. The ATtiny in turn pulses a MOSFET to control the sink current to a LED array. A simple design
Adam has posted the Eagle files
as well as some stellar photos and a video
of the box in action. If you've ever wondered how to do audio and LED control with a microcontroller, this is a great example project. Adam's got a great electronics related site as well so be sure to poke around.
Do we also mention we're completely enthralled he re-used his SparkFun box for a greater good? Nice job Adam!