The non diegetic (doesn't come from the film) sound adds tension, dread and excitement to the atmosphere of some scenes.
In parts of the film, diegetic sound is amplified.
E.g. in the scene before the Xenomorph kills Brett, we hear sounds of clinking chains and dripping water ( which you wouldn't normally hear that clearly).
Deep bass sounds are used to create fear in the viewer, adding an uncanny (mysterious) effect to some of the scenes.
Sound is crucial to jumpscares.
Loud, non-diegetic sounds are added to tense scenes to frighten the viewer- a conventional aspect to horror films.
A frantic, loud score is utilised to add excitement and tension to key sequences, specifically in the last 20 mins of the film- something conventional of action, horror, thriller and sci-fi films.