As you can see, this scheme is very simple. The use of a dual operational amplifier type NE5532 is a large part in simplicity, although it was quite possible to simplify even more by using a single transistor.
Polarization of the microphone
The electret microphone is fed Due to the resistor R1, 4.7 K. The value of this resistor should be adjusted (increased) if the power is chosen higher than the one proposed here (9V). The capacitor C1 prevents this voltage reaches the microphone in the first op amp, the reference voltage can be shifted and a problem of distortion of the high levels (asymmetrical clipping).
Small electret microphone gain
The op amp provides a first amplification of 10 (20 dB), this value is equal to the ratio of resistors R3 / R2. The second op amp is mounted in exactly the same way as the first, and also provides a gain of 10, and the ratio R5 / R4. The overall gain is equal to 100 (40 dB). If you want to be able to vary the gain, simply replace the resistor R5 of 100K with a 220K potentiometer in series with a resistance of 10K. The overall gain in this way can vary from 10 to 200.