Most speakers are basically a linear motor: there's a coil (the stator) and a diaphragm with iron in the middle (the "rotor"). When you apply an AC waveform to

Most speakers are basically a linear motor: there's a coil (the stator) and a diaphragm with iron in the middle (the "rotor"). When you apply an AC waveform to the coil, it moves the diaphragm in and out, producing sound. But it also works in reverse: moving the diaphragm in and out with sound produces a voltage in the coil. So fundamentally, a speaker and a microphone are really the same thing, just wired differently (and usually scaled differently: speakers are typically larger, and microphones typically have tiny diaphragms to be more sensitive).