This creates a slow, quiet, rainy-day atmosphere in the films, it is more of a film convention rather than a trailer convention though it can be used for as a change in atmosphere e.g. a scene in a bar where everybody is drinking and smoking cigars in expensive suits, or a romance scene. The purpose of this genre of music in Noir films was to create a gloomy and pensive atmosphere, to demonstrate the mood of the film to the audience.
Silence can be used in many ways:
To build suspense, highlight the significance of an event/give something meaning.
Create a numbing effect during a scene packed with action e.g. fight scene in Fight Club.
Halt the happiness in a film (e.g. all of a sudden everybody stops cheering)
A follow up to a loud moment in the scene to provide a dramatic effect.
It is important to keep a balance between silence and sound, the purpose of sound is to get an emotional reaction, so use it during important events e.g. a character dies, or all hope is lost etc.
There isn't much to say about off-key music other than it creates a disjointed effect and it serves a similar purpose as melancholy jazz, it could be used to connote the menacing society in the film, or the pessimistic view on the world, that everything is wrong and out of order.
Sudden sound works in line with silence, it is commonly used for jump-scares or sequences of action scenes. The sudden sound is conventionally a string orchestra, the harshness of the crashing chords and the abrupt shifts in pitch create a sense of fear in the audience. Sudden sound such as this is used to intensify an emotional reaction to a scene.
Voice-overs are used to narrate the plot or introduce the cast and their characters. Voice-overs are dying out but we intend to revive the old fashioned sound quality of the 50's. We won't go through the effort of buying a vintage microphone, instead what I plan to do is to record the dialogue and change the EQ and filter of the recording, and play it over a vinyl crackling sound to give it an authentic 50's feel.
This form of sound is used to resemble the time and setting of the narrative. Conventionally in Noir films that is pubs, bars, or busy streets. Noir narratives often include gangsters or some form of antagonistic characters, it is conventional for them to be introduced in a bar or pub, creating some kind of conflict with other characters.
Foley sounds are sound effects which are created after the production of a film, if something doesn't sound just as good as expected, you can re-record it or create something better. Foley sounds often include the small things suck as creaking doors, breaking glass, screams, footsteps etc.