Audio is translated into an electrical signal by a device that produces alternating current at the same frequency and intensity (voltage) as the sound waves hitting it, a microphone.
The voltage of that analogue audio signal is measured 44,100 times per second on a scale from -32,768 to 32,767 and stored as binary.
That binary does not go onto the disc directly as burnt area = 0, as many sources will tell you. The change from a pit to a land or vice-versa is what signifies a 1. It is also modulated to allow the player to read it more easily and encoded in a way so that the disc can still be read even if there are many bits that cannot be read due to a reasonable degree of scratches or dirt.
That finalized set of binary instructions is fed into a laser that etches the pits into a glass disc coated with a polymer that falls off glass discs easily when hit directly by a laser. It is then put in a vacuum chamber, coated with nickel, and a negative copy of that is grown by the complicated process of electrodeposition.
That negative copy is used to mold a disc of clear plastic with all the pits and lands. After it is molded it is coated with aluminum and another layer of plastic on top of the aluminum.
You put it in the CD player and the sensor that measures how much light is reflected by the laser sends that data to a little computer that reverses those modulations and error-correction encoding and sends that data to a circuit that takes the now-raw binary data and turns it back into an analogue signal that is amplified and sent to a speaker.
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