The audio amplifier circuit diagram is shown above. Control pin 5 of LM555, is driven by T1 BC547 BJT amplifier. Pin5 of Lm555 is biased to 2/3 VCC and if you look at the LM555 internal circuit diagram you will see that this pin is an input to a differential amplifier Q4. Varying voltage at the base of Q4 modulates oscillating signal of the LM555 timer. Changing the voltage at pin5 changes the width of the output pulse train, irrespective of value of RC components in the LM555 timer circuit.
The speaker can not respond to a oscillation high frequency and when voltage at pin 5 is zero, speaker doesn not produce any sound. When signal is applied to T1 it is fed to pin5 of LM555 changing voltage on that pin. The output PIN 3 is modulated due to this voltage at the control pin5, and speaker detects this DC component of output pulse and produce sound. VG1 voltage generator can be replaced with a microphone and R5 will be the biasing resistor.