Nick has a nice post about visualizing sound on an Arduino. He uploaded the Processing and Arduino code so you can try it out yourself. This looks like a good place to start learning about the Minim library and Processing.
I modified another old Processing program of mine to graphically react to sound using the Minim library. The program outputs the sound level to an Arduino connected to a breadboard, creating a sound level meter using LEDs. The code is a bit sloppy (and uncommented!) as I quickly tried to prototype these ideas.
More about Visualizing sound with an Arduino
In the Maker Shed:
Bare Bones Arduino Board Kit (Unassembled)