I've examined Atari 2600 sound, I tried writing music on it (using Paul Slocum's sound engine) and it's not very easy to work with. It does have some very strange waveforms. Few systems actually have distortion settings for the square waves, some waveforms are kinda like a cross between a square wave and noise. It sounds cool as hell, but the problem is there isn't much frequency control. So there are many notes that it can't play, and some of the ones it can play are out of tune. (it get's even weirder, the frequencies played for each distortion setting is different, I think).

Atari 2600 couldn't play NES music, but the NES could play Atari music - minus the characteristic waveforms (it's not as fun anymore, then).

A similar soundchip that easier to work with is Atari's POKEY (used in their 8-bit computers and many arcade games). I had some fun playing around with this tracker:
http://www.infos.cz/raster/atari/rmt/rmt.htm