If you want a software solution, it wouldn't be too hard to setup a linux box with a sound card. Here are a couple I've found for a project I'm working on, I haven't gotten to the point of the visualization pieces yet, I've only slightly researched it. I'll be trying Synaesthesia first I think, as it's one of the only ones with half decent documentation.
I have moodbar setup already, and I have a nice ruby script which will generate png images to the filesystem... if your interested let me know.

Synaesthesia
Fische
Moodbar