Measuring the mic signal of a computer headset
An electret microphone on a headset is typically wired up thus: 



Measuring the signal between V+ and Output (or Ground and Output) should give the sound waveform from the microphone. The resistor from V+ forms a voltage divider with the on resistance of the MOSFET within the microphone capsule, with the current through the MOSFET, and thereby the voltage at the Output, varying with sound. 

If measuring between the supply line before the bias resistor, and the Output in the above schematic, shows no waveform, the probes are probably loading the microphone too much. Electret microphones have very high output impedance, so a lower impedance probe would attenuate the signal to nothing. 

Also, the voltages involved are small, in millivolts, so it would be useful to set the oscilloscope to a more sensitive range.