Copyright Question

[Cranky Old Man]

It's illegal to use movies soundtracks since their license does not allow the content to be reused/modified for free.(most)

Well, a lot of rap music use sampling without asking for permission. They just can't claim copyright for those songs. There was a case where The Fugees wanted to go after somebody misusing their song, but they couldn't because they had in turn used samples from Enya, so they didn't have the copyright.
In other words, as long as it's a remix of the song, or it's only a small snippet from the song, you can get away with it.