The 20K variable resistor is used after the mic audio preamplifier circuit to adjust the output from the first transistor or volume.
The circuit can be operated from 9 to 12 volt DC. You can also operate the circuit with lower voltages like 3 or 6 volts. But it will not provide much sensitivity as it will provide with 9 to 12 volts.
This circuit can also be used with any electronic project which requires sensitive microphone circuit.