Nick Bertke aka Pogo is a master at creating musical mixes from movies. Take Upular as as a great example. It is comprised using chords, bass notes and vocal samples from the Disney Pixar film 'Up'. As he used to work for Pixar I guess he gets special permission because he has been asked to do others. I also recommend his great remix of Snow White and the Seven Dwarfs called 'Wishery'. See Pogo's website here and his YouTube channel here for more great examples of his work.