Sometimes when I direct scenes - or visualize scenes in my head I have a soundtrack running. I do this when I write too. I find that a song will set a mood or a tempo even if it really has nothing to do with the material at hand. It's mostly the music. It probably goes along nicely with my idea that a film is like a Symphony. It's a long an complicated theory about how each musician has his own skill that lends to the overall voice, the way individual technicians, actors etc have their own skills that lend to the overall voice of the film. There's a whole lot more too it, but being as I've had the SAME DAMN COLD for two months now, I really don't know that I can put it together coherently enough.
Oddly, though I frequently have music playing in my head, I'm not really good at writing with music on in reality - though I can tune it out, so to speak - while I'm working at Java Man - the little coffee shop I frequent at the beach. See - no coherency to this at all.