How to measure microphone output impedance
Of course. First you must have a repeatable amount of sound. Measure the output amplitude without any load (a high-impedance load, like a o-scope probe, is OK). Next load the output with a resistor equal to your best guess of the output impedance, let's say 1k. Measure the output amplitude again. If it is half the original value your guess was correct. Otherwise solve the equation

B * 1k = A * ( 1k + X )


where

A = open amplitude
B = loaded amplitude
1k = the load resistor you used
X = output impedance


Note that, apart from the requirement for a repeatable sound level, this is the same procedure you would use to find the impedance of any voltage source.