This circuit can be used to remotely monitor a loudspeaker, alarm, or audio source for presence of an audio waveform. It can also be directly connected across loudspeaker terminals used as a peak indicator.
If you need to monitor some audio signal at a location that is either soundproof are too far away to actually hear the signal then you can use a circuit similar to this one. A remote indication that the audio source is active is provided visually, here in the form of a LED. Referring to the above circuit, the 10k preset is a level control which should be adjusted so that when the source ( audio signal or alarm ) is producing the desired amount of noise, the LED lights. The input stage is an inverting x100 amplifier made with a 741 op-amp.
Read More Source: Andy Collinson
http://www.mitedu.freeserve.co.uk/Circuits/Misc/audio_ind.htm
