> 1. Smoothing out PWM to a decent analog voltage
> 2. Modulating that voltage on/off according to the PC speaker input
> 3. Amplifying the whole thing

If you don't mind post amplification it's easy.

Provide audio source never below ground.
Pass through a resistor - say 10k.
Chop output of resistor to ground with 1 transistor driven by PWM.
Feed resistor output (reduced in amplitude by PWM duty cycle) to a 
suitable filter (possibly a single R and C and thence to amplifier.
The amplifier could be a single transistor and could be part of an 
active low pass filter with gain.
If you can afford to use an op amp section or a few here it becomes 
extremely easy to do reasonably well.
An LM358 (dual) or LM324 (quad) would suffice.

If you have two PIC pins available and spare processing power you 
could feed tone to one pin. and out put PWM multiplied output from 
other pin.


        RM




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