Well music and sound can make a good and scary atmosphere,. Horror themed films usually have distinct themes so sound could be something that gives higher rating depending on factors like jump scares etc.
Then there is the tone, both sound and tone was different in the two films despite the one trying to have the same kind of build up, In alien we know its going to be a creepy, by the ambient sounds and atmosphere it gives, in avp we get the sense of an adventure film from the tone and sound.