The CD release of the Hitchhiker's Guide to the Galaxy radio program notes that it's mostly the original recordings, except for the music. They didn't have the rights to release the music on CD, so they substituted in different music. If you want the same program that was broadcast over the radio, you need to buy it on cassette, where apparently they do have the rights to the music.

Something has obviously gone badly wrong in the world of IP licensing. Somehow I imagine that movies don't license different parts of their soundtrack differently to make sure they can never be rereleased.