Typical electret microphones have the DC voltage specified, and the load resistor, too.
If we assume the DC circuit is correct, please reduce the audio "pump" power by adjusting source speaker distance to 1...10 meters while observing the distortion. If it is caused by audio overdrive, it will decrease with lower audio power.
To check the audio source distortion, best is to use a calibrated microphone as a reference.
I would suggest to use an audio fork tuned to "a", this is said to have no distortion.