two different stages. First stage is a high gain microphone preamp circuit and second is a single transistor amplifier to drive the headphones.
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.  

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The figure below shows a schematic of a very interesting sensitive mic circuit which will boost faint sounds from distance. The circuit is quite sensitive and provides good gain to weak audio signals.
The circuit is built around two 2N4401 NPN transistors and contains

Sensitive Microphone Circuit
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