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Hi.. Good Day to all.. can you please help me explain how this circuit works??

The Electret mic needs power to operate. C1 AC couples the MIC signal.
T1, t2, t3 and T4 effectively amplify
Th voltage across R11, D1 and the inherent diode drop in the Triac as well as the minimum current determine when the triac turns on.

A sound of a specific intensity turns on a lamp.

T3 and T4 is not for switching??

McFLY said:

T3 and T4 is not for switching??

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It looks like T3 and T4 are part of a limiter.

limiter?? what do u mean for that ??but t3 and t4 switches also signals that trigger the triac to on??right??

As the Collector of T4 goes up, the base of T3 will be pulled low.

The triac only needs a brief pulse to turn it on. It also must be of a specific current. It also has to get past the gate diode. The datasheet is here: https://www.electro-tech-online.com/custompdfs/2011/12/BT136_SERIES-5.pdf

You will need 0.7 V across the gate + the 0.7 across D1 and you need between 5 and 30 mA of gate current depending on the quadrant of operation. So R11*I(gate) has to be bigger than 1.4V and has to cause 5-30 mA of gate current to flow.