Heh, that's probably a bit too much to as from the user. Personally I know very few persons who even own a microphone, let alone a stereo one...
There could be a simple "audio calibration" program in the system settings though. Play a few pure sine waves of various frequences and let the user raise the volume until it starts to distort. Then for line-in/microphone they could experiment with different input volume levels. The values chosen would then be used as "100% volume" in the rest of the system.