Described here is a very inexpensive solution to many phono-controlled applications like remote switching on, for instance, or activating a camera, tape recorder, burglar alarms, toys, etc.
http://www.electronic-circuits-diagrams.com/audioimages/audiockt9.shtml
An infra-red or wireless remote control has the disadvantage that the small, handy, remote transmitter is often misplaced. The sound operated switch has the advantage that the transmitter is always with you. This project offers a way to control up to four latching switches with two claps of your hand.
http://www.electronic-circuits-diagrams.com/remotecontrolsimages/remotecontrolsckt4.shtml
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