The software is of course on my work machine and remote from me at the moment. I'll try and post it tomorrow.
However all the sound generation is in software. The sin wave just uses a sin function. It's Fm modulated by simply adding a slow sin to the angle sin(theta + modtheta).
The final value is simple out=sinvaluesquarevalueADSRvalue
I use digital outputs 7->2 as the value output. That goes into the D/A converter (Based on this http://www.allaboutcircuits.com/vol_4/chpt_13/3.html but with no op amp following it). This goes into a TDA7052 amp using the simple circuit from the datasheet. It's dirty, but for this app thats fine. I might need to add a volume control as I think I'm overloading the amp at the moment !
Andy