Hmmm, I'm working on a very similar thing, but I've decided I'm going to try and figure it all out myself, so I won't look at your technique. 
I'm aiming for 4 voices, but with 16 step 4 bit waveforms. I'm also using a separate timer to control fast PWM to run thru a lowpass filter in order to get an analog output.
Monophonic is working great, I posted it in exhibition, but still figuring out polyphonic sound.