Depends on the headphones and their power rating. I think a common ohmic rating for headphones is 16 ohms, they only put out a few hundred miliwatts of power so in the neighboorhood of 1-2 volts maybe higher on a high quality set.
The microphone question is a little harder to quantify because most headsets use electret microphones and they're active devices.
On headsets, are you familiar with the decibel output of headphones, or the efficiency of a microphone to convert decibels into volts? Or could you help me search for two tables describing those? For anyone reading this, I'm still interested in collecting more of the voltage data described in my original question also.
Panasonic makes several very good electret microphones, with great frequency responses from 20Hz up to about 10kHz where they start to roll off. Most inexpensive electret mics (<150USD) are based off of them, and the capsules themselves only cost a few dollars.