You should measure these dc voltages in circuit
Tell me the values A, B, C, D and E
Take out wire D from potentiometer side
and when power is on tap this wire with finger tip , you should hear 50Hz hum from speaker.
Mic. amp. test :
you should find small audio amp. e.g. active pc-speakers
connect this test amp. input gnd to gnd
input hot wire
via 20...100nF capacitor
1. to mic. hot wire tap mic, what you hear.
2. to transistor base, tap mic. , what you hear.
3. to transistor collector, tap mic. , what you hear. (Is this louder = amplified in transistor)
KAK