It is truly a mystery how sound comes out from those disks. Of course, we know that sound comes out from a CD because sound is recorded on the CD, but the question is how the sound is recorded and how it is converted back to sound.
First, to record sound on a disc, the sound is converted into an electrical signal. The electrical signal is then burned into the surface of the disc in the form of tiny, invisible indentations. The indentation is approximately 1/1000th of a millimeter. These tiny indentations are burned from the center of the disc outward in a vortex-like pattern.
When sound is produced, the spinning disc is illuminated by a laser beam from below. The laser beam is reflected differently from the shiny surface of the disc to the hollows. This reflection is converted into an electrical signal, which is then converted into sound.
There are billions of these tiny dimples on a single disc.