I work for Analog Devices, who makes MEMS microphones. I have written an application note to describe how to use a MEMS microphone in a circuit that was originally designed to use electret mics. Along with the MEMS mic, it requires a few additional passive components to remove the bias from the signal and provide a dc supply to the mic's Vdd pin. http://www.analog.com/static/imported-files/application_notes/AN-1181.pdf. I hope this is helpful.