Oh, it's certainly allowed. The problem with a lot of the new, cinematic soundtrack stuff is that (a) it's really complex, ( it's already heavily arranged, and © there's no way to do it other than by ear, i.e. there usually aren't any easily-accessible MIDI files. A remix of a modern soundtrack song would probably focus in on one single part, and then expand on that a bit. Heck, I think I could do an entire album just from the first track of the Starcraft II OST.