I made a super simple music visualizer using Processing, Arduino, and a RGB LED. As this a basic project involving some sound I'm using only the red led on the RGB led to change brightness based on values in the audio buffer.
The processing sketch controls the sound being produced (i.e. playing and pausing it); and sends the values to the Arduino which changes the brightness of the led. Sounds simple right? I've put up the instructions at http://theblubot.blogspot.in/2014/01/simple-music-visualizer.html