The diaphragm of a condenser microphone is the negatively charged plate of a capacitor. This Interactive Java Tutorial shows a cross section of the diaphragm.
Sound waves cause the diaphragm to vibrate at the same frequency as the sound waves.
As the diaphragm moves closer to the positive plate of the capacitor, the electrostatic attraction between the plates increases. This causes current flow to the diaphragm. As the diaphragm moves away from the positive plate, the electrostatic attraction between the plates decreases. This causes a flow of current back to the positive plate. The current alternates at the same frequency as the sound waves and travels through a mixer, then through an amplifier, and finally to a speaker.