Here is the link to filmsound.org. It's a terrific site with articles and interviews dealing with one of the most underappreciated and misunderstood part of the filmmaking process.
When you watch a film you divide the experience two ways, visual and aural (at least until smell-o-vision arrives). Most of the effort put into films lean on the visual with very little thought or time given to create a dynamic and meaningful soundtrack. Composers are often hired after the film has been shot and often forced to recreate a temp music soundtrack used by the editor and director (it's very dangerous to use temp music - you end up tying yourself to it).
The sound department along with the composer and musicians provide the emotional underpinnings that are so important in making a film. Music can make the difference between a laughable melodramatic scene and a powerful, emotional scene. Tone is such an important part of filmmaking and a lot of that tone is generated on an aural level.
Take this for example.
Granted, they are playing with the story and it's an extreme example but, other than the hokey voice over, it's the original footage from the disturbing and eerie Kubrick film. It's a simple change in tone and we are all having a laugh. It's that easy to screw up a film.
Here is the original. The 'music' in Kubrick's trailer is much more like soundscape then what we consider to be a tune or melody. The spectacular Bernard Herrmann was a pioneer of this use of music. It has everything to do with tone and emotion, a revolving uncomfortable piece that lacks progression or resolution.
Postscript: Kubrick liked to use classical music as his temp music while editing and then would end up using it in the final film thus saving some poor composer from having to imitate the un-imitatable (I may just made up a word).