The Transistor T2 was wired to work as a switch in the above circuit ,so whenever a signal reaches the base of T2 current flow will takes place from the collector to emitter of the Transistor T2.
In this circuit NE 555 was wired as a monostable multivibrator in which a negative trigger to the pin 2 of the IC gives out high signal and positive trigger gives low signal in the output, read detailed description working of 555 Monostable mutivibrator . So when the transistor T2 is switched on by the incoming signal, it permits current flow from C to E thereby giving a negative trigger to the NE 555. The negative trigger in turn changes the output of IC1 to high state powering the Piezo buzzer to produce sound for fixed duration. The duration was decided by means of R and C values connected to the IC 555. The time duration was given by the formula
T=1.1 x R x C
Connecting relays at the pin 3 of the IC will enable you to drive high loads and this could allow the above circuit to be used as sound or noise sensor.