In general, you tap off a sample of the signal. Then you rectify it (try a half wave op amp rectifier circuit.) Then you go through an circuit to produce an average so that short sounds do not trigger it. This can be a series R and shunt C or it can be an op amp integrator with a resistor across the capacitor. Then you go to a comparator with a threshold that can be set (trip amplitude).
I'm trying VOX for mobile phone and a BT-adaptor.Thanks to GaAs FET for showing the good link. I've got an idea. A PIC with comparator may be
used for the voice detecting and act as a switch.
JaBa if you do come right, please let us know were this circuit can be viewed. I want to do basically the same, my PIC circuit work, but the voice circuit is a bit troublesome.