Hi, Project 21 in my book '30 Arduino Projects for the Evil Genius' is a VU meter that uses a microphone for input, but you could easily adapt it.
If you want the source code for the sketch, you can get that without buying the book at http://www.arduinoevilgenius.com
There was a post on this forum somewhere where I posted the circuit, but I can't find it. You may have better luck.
Simon.