Influence of sound in thriller sequence

As a first sequence I choose clip called 'Ends of Deceit', which is a opening scene to a thriller movie made by Hurtwood students. 

This clip starts with a few seconds of credits accompanying by non - diegetic ambient melody, in order to create tension and set a viewer in a insecure mood. It is also a sound bridge to the beginning of the action when camera is panning in the room to show the location of a action. The purpose of the sound bridge is to keep the audience in the same mood and prejudice. 

Then we see a girl in her bathrobe coming into the kitchen with her dog. The observer sees and hear door opening so it is a diegetic sound.

The mysterious melody is still playing in a background but at the moment when the actress comes in the additional sounds begin. Her footsteps, or rather scrapes are a diegetic and pleonastic sound. Due to her clothing we can assume that it is morning, so the scrapes can suggest that she is tired or she has just woke up.  Girl opens the door which is a diegetic sound and lets her dog out. In this point the gloomy soundtrack ends, what make the audience more concentrate on the following sounds. From that moment the audience can see only the dog. His footsteps, which are diegetic and pleonastic are imposing his importance in this clip. For the next 40 seconds the audience can see the dog running around in the forest. We can hear simple, natural and delicate sounds of nature - wind, birds, movement of leaves - these are all non - diegetic sounds. They influence on our suspicions, now it seems like nothing special will happen, every sound is neutral. The audience abandons the mysterious and scary perception of the clip. On the next shot we can see the dog in front of the house. He is getting inside by pushing and scratching the door, which are diegetic sounds. He is running to his bowl with food and starts eating. Here the tension starts to build up again. While eating the ambient melody starts to play one more time , which makes the audience questionnaire what is happening. Sound of eating, which is diegetic sound and soundtrack set the audience in the uncertain feeling what is going to happen next.  Subsequently we can hear dog's footsteps, which is a diegetic sound, he is going to other room and suddenly the ambient sound start to play parallel with footsteps and the audience can see the victim of the murder - girl is laying on the floor with her head in blood. The sudden appearance of deep and intense sound with the view of killed girl shocks audience. Afterwards dog is walking next to the victim squealing, which is a diegetic sound. It makes the audience feel helpless and pity. 

After watching this clip i thought that the amount of sound is balanced. It did not bulit up tension throughout the whole clip which i liked because at the beginning the audience is waiting for the plot twist but suddenly it dropps and lives the observer with confusion. The plot twist takes place at the end when the audience does not expect that.