Every speaker is a microphone. This is because every speaker has a magnet and coil. A pair of wires connects the two. This enables the speaker to act a device which is used for outputting sound. Electricity is converted to sound. It is possible to reverse the process and making it possible for the magnet and the coil in a speaker and enable it to pick up sound that is in the air and turn it into a microphone through the electrical signals in the wires. To do this you can take a speaker and connect the 2 wires to the input which says "mic" on a device like a tape recorder, amplifier or even a computer. You will see that the speaker will act as a microphone and will pick up sound from the atmosphere.