analysis of sound in thriller opening

There is not much sound in this opening sequence, however the silence is equally important in building the suspense.
At the beginning, in the establishing shot we hear the diagetic sounds of birds and the outdoors, letting us know it is an ordinary quiet night. We also hear the letter box go, telling us this is important to the storyline. When in the house there is silence, only the sound of the girl drawing the curtain.
When we are outside again the diagetic sounds return, except this time with the added non-diagetic sound of the attackers heartbeat,  bringing an increasing sense of fear.
At 50 seconds, a non-diagetic soundtrack begins to play quietly. This is strings dominated as it helps to create tension. Now all that can be heard is this soundtrack and the heartbeat. Gradually the heart beat becomes louder and more drums are introduced to the soundtrack as the attacker gets closer to the victim. Both stop suddenly when the victim to be, turns and we see her reaction shot.
The lack of dialogue in this clip, makes it unclear for the audience, creating a sense of mystery.

Right from the beginning there is quite a loud, threatening non-diagetic soundtrack, played by an electric guitar. The style of this music creates a sense of urgency. However we also hear the diagetic sound of someone's running footsteps, adding to this atmosphere. In the soundtrack, in the background there is an eerie voice, making the clip seem more sinister. The footsteps are not constant, only returning when we see the character themselves running. Added to this we then get the heavy breathing of the character, despite never seeing their face. This creates a sense of mystery.
The soundtrack then dies out and we are only left with the heavy breathing as the attacker approaches their victim. This lets the silence create the tension, until the very end when the victim spots the attacker and the soundtrack comes back in, as they fall to the ground and we are left with a closing high angle shot of the attacker. The soundtrack reinforces the menacing nature of the attacker.
This also has no dialogue, confusing the audience with the sense of mystery.