How to create visual representations of sound?

Another basic place to start is the jit.catch~ helpfile (and there are lots of others that address this too). It's a graphical representation of the signal itself, similar to the signal scope object, so it's straightforward. For more esoteric or abstract representations you need to determine what you're looking for (and a lot of times this changes as you experiment).

Get into GL a bit, perhaps, so you can use parameters to create lines, shapes, etc., all with color and blend / alpha (need blend_enable 1 for transparency). You can do a lot with GL, though it'll eventually eat up processing depending on the final product. If you're playing back MIDI notes rather than signals, you'll probably have more options with parameters -- pitch, velocity, instrument values etc. can readily be mapped (as opposed to signals which need some analysis first). However, signals have the potential to be a lot richer and have lots of filtering, delays, etc. applied to them, a whole other world of experimentation there too.

The Max / Jitter environment is perfect for this kind of stuff... pretty much made for it, in a lot of ways. And yes, you could do some stuff without Jitter, but it will provide a TON more cool options for this specific project, so it's highly recommended.

--CJ