Do movies use original master tapes for soundtrack songs, or do something else?

I know there are some film industry people around here, so I feel confident that there will be some good info.

In Quentin Tarantino’s Death Proof, I noticed that T. Rex’s “Jeepster,” played in a bar scene, sounded significantly better than any CD copy of the song. It actually sounded even more dynamic, not just compressed and sexed up with EQ. Every aspect of the song sounded more present, three-dimensional, and lifelike than any CD or vinyl version of it I’ve heard.

When someone is making a mainstream Hollywood film, what do they do for soundtrack songs? Are the original master recordings used, or are the filmmakers doing the equivalent of ripping the song from a CD and loading it into their editing program for the soundtrack?

Bump, because I found this to be an interesting query.