Using the audio input port as a voltmeter
Audio inputs are normally AC coupled (there is usually a series capacitor to block any DC component in the input). Typically this means you won't see much below about 20 Hz and you certainly won't be able to measure DC or slowly varying signals.

As for sensitivity, typical "live level" inputs expect a signal of 775 mV RMS which corresponds to 0 dBV. Microphone inputs are usually more sensitive than this, but there is no "standard" sensitivity and the input hardware often has some kind of controllable gain stage prior to the A-D converter.