My friend is a band, and for on-stage visuals, I've made him a (trashed) compact mac turned into a

. He's not happy with it, So I've decided to get a Classic II, hook up the audio out from his mixing board to the audio in, and write an oscilloscope program.
How hard would this be? I've obtained a copy of Metrowerks Codewarrior, and am ready to get cracking. I have some C experience, probably a little rusty, but doable.

What am I looking for in terms of hardness of what I'm about to set out to do?