A Cd contains tiny buckets of ones and noughts. When the disk spins, the centrifugal force hurls the ones and noughts off the surface where they are collected by a tiny suction tube which directs them to them to the AD chamber.
The rate and quantity at which they strike the walls of the AD chamber cause it to emit vibrations which are sent along voice pipes to your speaker system. The faster the CD spins, the harder they hit the walls, and so the louder they sound. As the ones and noughts are very tiny, there are billions of them in the buckets, but eventually, if played often enough, the disk will run out.
Philips (who invented the CD) had hoped to provide a network of filling stations where you could refill your empty CDs but the music industry wanted to charge such exorbitant royalties that the plan was dropped.