Working of the LM-358 - microphone circuit
More than likely, the microphone has an output impedance that is high (drain of a JFET = current source) and so the two 5.1 kohm resistors (that connect to the microphone and bias it) form the effective input resistor to the inverting input. If you analysed the circuit you'd realize that the two 5k1 resistors are to be regarded as in parallel (for AC analysis) so the effective input resistance is about 2.5 kohm.

So the small signal AC gain is 1 M / 2.5 k = 400.

This will be the gain up to about 2 kHz then the op-amp (LM358) will be unable to sustain this gain at higher frequencies and you have a roll-off in gain of 6 dB/octave.