The ticking clock, reverberated, suggests passage of time - mortality, time passing in an empty space (why are we not in the same space as the time time which is passing? Are we dead?) - urgency.
Music box - evokes childhood anticipation of entering the unknown - dreams, nightmares, fear of insomnia, disrupted rhythms
An entity moving towards the camera places the viewer subjectively within the mis-en-scene/story world, in doing so amplifying the viewer’s empathy with the onscreen events.
Chains rattling/metal upon concrete - impact upon or friction with hard, often wet surfaces, evokes the antithesis of a warm, cosy, soft bed
There must always be a dream sequence that taps into the fractured non-linearity and ironic juxtapositions of surrealism (which as a movement aspired to recreate the conditions of dreams).
Sustained dissonant strings - are not easily musicologically delineated in the moment and densely pack compound emotions (unlike the soaring, simple, unison melodies of melodrama or romance). Also strings through being instruments of sustain imply a plateau - they’re topographical and suggest space rather a staccato or percussion instrument which serves to indicate/underscore a moment in time.
An entity moving from one dimension into another - ie from a television screen into the “real” world - threatens and intimidates the human impulse to compartmentalise. We compartmentalise in order to organize the chaos of the universe and it’s attendant data into a tidy stratagem and thus attempt to not go insane. When a Lynchian cypher flickers from Box A into Box B, it disrupts our method and we panic.
High-definition and CGI are heinous within horror - successful horror achieves cognitive disruption through half-perceived entities and events - the imagination of the viewer colours in the half-perceived and thus brings with it the deeply personal fears of each viewer. CGI too often lacks or fails to simulate mass and we need to believe Pinhead is in the room with us in order to truly and desperately wish he wasn’t. In-camera and optical effects succeed because of their haptic qualities.
Strobes and flickering lights are effective because they make us work harder to unpack the contents of the image-space, and because of diminished visibility reduce the time in which we can do so, inducing a sense of urgency. Also the layering of frame rates - that of the film itself, that of the flickering element, and that of our brain waves in observer-decoder mode - created rhythmic dissonance, which is uncomfortable.
Horror needs to be character-driven. Mindless mutilation is not the domain of successful horror but that of the United States military.
Bonus points for upsetting the moral sanctity of the viewer by implying their complicity in the disgusting and violating acts perpetrated onscreen. Make them doubt their emotional and moral fidelity. Make them query their religion, their social and familial allegiances.