Why to military headphones and microphones have an impedance of 8 ohms, and civilian ones have an impedance of 150 ohms?
Wild-assed guess: so the civilian ones automatically decrease the sound volume when plugged into the speaker circuit. It’s a nice feature for your headphones to have a similar apparent volume when you’re blasting Pink Floyd and you hear your mom’s car in the driveway and so plug in the 'phones. Military headphones are usually the primary way to listen to a signal, rather than a supplemental or occasional way.
(This post from a guy who couldn’t afford to buy headphones so he put 8-ohm speakers from old transistor radios into tuna cans. You need to turn the volume waaaay down when you put those on).
Civilian headsets are also used primarily to listen to a signal. (Pilot, remember. 
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I guess I also need a quick primer on impedance. I know that a headset with one type of impedance used on the other type of radio results in a very weak signal, but I don’t remember which is which. Not that I have a need to use a military radio, or a military headset on a civilian radio, but why the difference? Is it because TTWWADI? (If so, is it the way it’s always been done? What about the early days when radios were often rare in aircraft?) Or is there an advantage of one over the other?
military had 40, 100, 250, 400 and 600 ohm impedances also in sets i’ve seen that were old.