Unfortunately couldn't locate the circuit that I was trying to find...it was really just an electret preamp like you'll find on the SparkFun board or around the web: http://www.sparkfun.com/datasheets/BreakoutBoards/Amplified-Mic-Electret-v14.pdf
And there's another circuit you might find useful. Trying to filter the audio in software might not work too well. I use this circuit to rectify the audio and do a peak detection. You could combine the two circuits by getting rid of everything to the left of R5 on the VU circuit and connecting the output of the electret preamp through a 10uF capacitor. Run the output into an analog input of the Arduino and you should be able to do sound reactive stuff pretty easily. You could use a single opamp chip like the LM324 to do everything, it has four opamps.
Maybe you can hack something with just a single diode and capacitor for the peak detection, but it won't respond to low levels, which will be pretty crucial if it's listening to ambient sound. In all cases you can't omit the electret preamp circuit, there's just no other way to get enough signal out of it.
Another option is to use some device that already has a good microphone and can output amplified audio. One example would be an iPhone, you can get apps that just input audio and play it out the headphones with volume adjust. This could also let you play your own music, if you split off some amplified speakers.
Good luck...time is short