Not sure about Mac, but on PC I've toyed with VST Plugins that do NES, C64 and similar noises. That's an option. Another one, though I don't know how the Tracker situation on the Mac is, is to check out some classic MOD chip tunes (files in .MOD, .S3M, .IT, or .XM format). They'd have the samples, and give good examples on how to make certain noises. Hope that helps.