The simply symmetry of speakers and microphones.
All sounds are produced by vibrations. With a permanent magnet and an electromagnet produced by current flowing through a coil of wire, speakers convert an electrical signal into tiny vibrations, which are conveyed to the surrounding air by an attached diaphragm. We interpret these vibrations travelling through the air as sound. Microphones use the same concept in reverse, with vibrations from the air causing a diaphragm, and hence a wire coil, to move, This movement relative to the permanent magnet generates a varying electrical current.