An earphone and a microphone have the same general concepts behind them. A microphone pics up the vibration from your voice and turns it into electrical energy, and a speaker does just the opposite, takes electrical energy and turns it into sound. The similarities end there, however, as a microphone has to be extremely sensitive, and cannot handle the voltages involved in producing sound. A speaker, however, is extremely robust, and will not be a good microphone because it isn't sensitive at all.