One of my biggest concerns was being able to process the audio signal quickly enough and display it with an appropriate refresh rate. The answer to my questions came int he form of the MSGEQ7 graphic equalizer display filter. This wonderful bit of hardware takes a single channel audio input and converts it to a DC signal displaying 7 frequency bands. Sparkfun has since re-released their Spectrum Shield, which would have saved some significant effort, but this was a good build nonetheless.
I was also concerned about the quality of the microphone, especially since nobody would be talking directly into it. The electret microphone breakout from Sparkfun ended up working well.
In general, the flow would be: