using headphones as microphone
In most cases, headphone transducers will also work as dynamic microphone transducers. However, they put out a VERY SMALL amount of signal which you will never see on a regular meter. No it cannot be "amplified" by simply applying a DC voltage (from a battery or whatever). You need a high-gain AC amplifier. Specifically a microphone preamp to bring a "mic-level" signal level up to "line-level" which can be seen on a meter. Typically 40dB (10,000 x) of gain.  There are hundreds of examples of mic preamp circuits online. 

Most small microphone capsules used with computers, etc. these days are electret condenser microphones which are quite different than dynamic microphones.