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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.
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.