Possible to read voltages with a DMM that's coming from an unpowered microphone?
If the microphone has a balanced output (TRS or XLR connector) you should not read any voltage between the ground (sleeve of TRS or pin 1 of XLR) and either signal lead, but you may be able to measure a small voltage between the two signal leads (tip and ring of a TRS plug, or pins 2 and 3 of an XLR connector) (I don't have a microphone handy to check - the voltage may be too low to measure with a DVM)