Just a guess, but I reckon you should be reading the microphone as an ac signal, biased to half Voltage, then when you read it the values will be probably equal either side of half your reading resolution
for example if your resolution is 1024, then the half way point would be 512, so you would get ac/sinwave values above and below this number.
to ensure you would get a good ac coupled signal, use a 1uf cap (to block the DC path) then bias the output of the cap with two equal value resistors, one going to +ve, the other to 0v