Production companies can choose music that isn't in the actual movie, and even stuff that was written for another film entirely. Recent examples of this include the Dracula soundtrack featuring in The Mummy Returns trailer, and the Crimson Tide theme appearing all over the place. Trailers generally use music from other movies because the film in question's score is not yet finalised. Even when there is a soundtrack available, it may not be suitable for the 90-second trailer, which requires different dramatic beats. The cost of using a piece like Dracula can run into tens of thousands of pounds, which is why many trailers use tracks bought cheaply from music libraries.