>> I'm trying to control the PC speaker (motherboard beeps etc) volume 
>> using
>> PWM (software-driven PWM from a PIC) but I'm not really sure now to
>> design the circuit.

Given PWM of a high enough frequency (20 kHz plus frame rate) you 
should be able to simply gate the audio signal with the PWM OR ground 
it if the supplying impedance is high enough. You will need some sot 
of electronic switching - quite possibly a single FET would do. If 
speaker is AC coupled it will be slightly more complex. Assume for 
simplicity that speaker is DC coupled and driven by a signal never 
below ground. A FET in the speaker ground lead could be gated by the 
PWM signal. A flyback diode would probably be needed across the 
speaker in series with a diode to *attempt* not to modify audio 
response too much. It would be easier if you could control the signal 
level at lower level and then amplify it for the speaker but that 
wrecks the simplicity.

See what sense you can make of that and ask more questions.

        RM 

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