When we hear sound, we are actually sensing air pressure changes. A microphone uses this principle directly.
Microphones have a small membrane that responds to pressure variations by vibrating.
This motion generates a small electric current. A small laser in a CD player reads these depressions. As the CD rotates, the information is translated into a sequence of signals. These signals are again interpreted was strings of zeros and ones, re-creating the original digital signal. The decoded numbers are translated back into voltages, and the resulting electric signals are sent to a speaker.