How many times have you wished that there was a simple way to turn on that sub-woofer or some other piece of audio equipment, simply by sending it a signal? This ability is fairly common in commercial subs and some other gear, but there seems to be a complete absence of circuits on the net, and they seem completely unavailable as an add-on device.
http://sound.westhost.com/project38.htm
In its simplest form, a voice-over unit is just a microphone and change-over switch feeding an amplifier, the output from the microphone having priority over the amplifiers audio signal when the push-to-talk switch is pressed.
http://www.zen22142.zen.co.uk/Circuits/Audio/v_over.htm