Was bored, got a little mad today and then found this while killing time at youtube.
This should answer your questions on why the soundtrack of Alien is what it is in probably more detail then you ever intended to get told.
Serious, if you are interested in music theory and how film and movie get married by the creators and why and how it sometimes can misatch (or not), then this is a good half an hour of time to invest in. I found it interesting.