This sensitive sound operated switch can be used with a dynamic microphone insert as above,
or be used with an electret (ECM) microphone. If an ECM is used then R1 (shown dotted) will
need to be included. A suitable value would be between 2.2k and 10kohms.
The two BC109C transitors form an audio preamp, the gain of which is controlled by the 10k
preset. The output is further amplified by a BC182B transistor. To prevent instability the
preamp is decoupled with a 100u capacitor and 1k resistor. The audio voltage at the collector
of the BC182B is rectified by the two 1N4148 diodes and 4.7u capacitor. This dc voltage will
directly drive the BC212B transistor and operate the relay and LED.
It should be noted that this circuit does not “latch”. The relay and LED operate momentarily
in response to audio peaks.
Source : http://www.zen22142.zen.co.uk/Circuits/