Electret Microphone Operation
The DC content of the output will remain the same with or without the presence of any acoustic stimulus. Inside the microphone is a JFET (usually) that takes a constant DC bias current: -



This means that the DC voltage dropped across the external resistor (up to V+) is also a constant DC voltage. When sound is received, the constant DC current is "modulated" by the sound and this superimposes an alternating signal voltage across the standing DC voltage on the external resistor.