Impedance of an Electret Mic
Measuring the impedance of a microphone is pretty easy.

You need a constant acoustical signal such as a speaker fed by a tone.  Mount the microphone in front of the speaker in such a manner that it won't move around as you are doing the measurements.  Standard measurement frequency is 1 KHz.

Apply the bias voltage to the microphone and measure the amplitude of the output signal.  Note that value.

Now connect a resistor and capacitor across the microphone.  Make the capacitor large in value - 1000 uF is massive overkill but ensures that capacitor reactance doesn't affect the measurement.

Now decrease the value of the resistor until the AC voltage across the microphone drops in half.  Alternately connect and disconnect the resistor from the microphone output to ensure that the voltage ratio is exactly 2:1.

Now remove the resistor from the circuit and measure its resistance.  That resistance value is the impedance of the microphone at that frequency.

Repeat the above procedure for each frequency desired.  However, I find that most electret microphones have a flat impedance frequency response over the range of frequencies that I am interested in.