Just quickly googled our question and this guy seems to know his stuff. More research is needed though.
Just to check.
But it does sound convincing...
"When someone talks into a microphone the movement of the diaphragm caused by the sound creates patterns of tiny electrical pulses. (These are wave patterns, such as you often see in audio software. There are hundreds of pulses a second)
Likewise the same pattern of electrical pulses put through a loudspeaker causes the diaphragm to vibrate and make the identical sound that was spoken into the microphone.
A tape recorder takes the electrical pulses from the microphone and runs them through an electromagnet that is touching the continuously moving tape. The tape contains magnetic material that alters its position depending on the strength of the magnetic field.
When the microphone creates a strong electrical pulse then the magnet will become stronger and create a different mark on the tape to a weaker one.
Conversely, when reading a tape, the movement of the differing marks on the tape past the reading head of the tape recorder creates electrical pulses that match those which were originally created by the microphone. When these pulses are put through a loud speaker you get the original sound that was spoken.
In the real world a tape recorder has to increase the level of the electrical pulses from the microphone (e.g. amplify them) in order to make them strong enough to affect the tape, and it does the same when reading the tape so that the loud speaker actually makes a loud enough sound."