That's depend on the contract. Mostly they get paid for each CD people buy, or they can just pay the artist (s) for the recording section and that's it. In my case I have had both ways. For me is better to get paid at the moment for the job I do, better than have just 00.05 or 00.07 cents for each CD. What about if people don't buy the CD? Now, would be different if you are recording for some important artist or music director and you are sure that CD would be 1,000,000 copies sold out!

They don't pay for the music you hear in the radio, they pay for CD sold or for recording section.

Hope this answer your question.