Why do I see 0v on electret microphones?
Electret microphones have an internal amplifier that requires a bias voltage at a few mA (or less) current; they have a moderately high impedance. Carbon button microphones are much lower impedance. Note that though a carbon mic is nonpolarized, i.e. it doesn't matter which way it's connected, electrets may require connection of a specific line to positive. Some electrets have a three-wire connection, one for ground, one for V+ and a third for output.

The schematic below should work with most two-terminal electret mics to drive a standard mic input on a PA, but you'd need high-impedance headphones or anther stage of amplification to drive a speaker.


from http://www.diyaudio.com/forums/instruments-and-amps/188392-buffer-preamp-circuit-electret-mic.html, modified