> I could use a digital potentiometer to vary sound volume,
> however if I could get the same effect by using the PIC's
> 10-bit PWM peripherals it would be ideal

I wonder if changing the duty cycle would change the tone
more than change the volume. Perhaps try something like
a constant sound output from the PIC and vary the gain
with PWM. For example rectify the PWM to control a
" proper" VCA like the MC3340 or digitally-controlled op
amp. Alternatively the gain of a transistor/FET. PWM can
also be used with a 4066. Effective R is the ratio of the on/
off times. The 4066 element could be used in a voltage
divider or op amp feedback path (quite a neat way to make
a VCF)

For source sound you might give Roman Black's 1-bit
method a go

http://www.romanblack.com/picsound.htm

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