I looked into using the chip from a nes, but i was unable to find any datasheets at all... I don't know if that is me not looking very well... there seems to be a few synths made from the C64 SID chip tho... http://www.ucapps.de/
the NES chip is a custom CPU with a built in synthesis section... I believe it is based on the 6502 and probably won't work as a stand alone sound generator.