Compressing and Summarizing sound:
In film, audio is often compressed and edited for a specific purpose. This technique is used in probably every single trailer or teaser for a new movie that is going to be released. To illustrate this for you, here is the trailer for 'Star Wars: The Last Jedi' where towards the ending you hear Rey stating that she needs someone to show her her place in all of the chaos and you see Kylo Ren offering his hand hinting that she is going to take it and join the sith.
However, in the actual film, it is really Kylo Ren that is offering Rey to join him telling her to let everything die and go with him. The sound is compressed in the trailer to cause the audience to doubt Rey's true goals and leads to the anticipation of what she's going to do as they mute out the "join me" audio and cover it with the sound of tension-building music. Here is proof of the real clip which is edited for the purpose of the trailer:
Eliminating material:
In most pieces of film, a director/ producer can decide if certain pieces of audio are flawed, repetitive, superfluous, uninteresting or irrelevant. If found to be one of these things, they will be edited out post-production either completely or will be re-recorded by the actors until they feel it fits perfectly. This has to be done precisely, if it is not the entire movie can be ruined and the audience will lose interest in the story line itself. Here is a video explaining why the audio tracks from 'American Hustle' were not perfected enough for the movie to be entirely successful and flawless.
To summarize this video, it basically states that the editing team did not focus hard enough on all scenes to notice small audio errors that can affect how the audience receives the film. Some things caused by the incomplete editing were vocal distortion, gain hiss, incorrect microphone placement and overlapping dialogue. All of these things can confuse the audience by preventing them from clearly hearing all of the words said by a character, causing them to focus on that and lose focus on the progression of the story line as a conversation between two characters can determine the shape of an entire plot, therefore if some of that conversation is presented wrong then the entire film is affected.
Recorded speech/ inappropriate language:
The directors, producers and writers must all make sure that the speech/dialogue in a film is completely legal, otherwise it will not be successful at all as it will not be allowed to be viewed publicly. This could be if profanity is used in a film that is meant for the age rating of PG or U (for all ages) in regular dialogue or music tracks, an example of this would be if a film with a U certificate wished to use a song like 'I Like It' by Cardi B, they would need to bleep out many of the lyrics as they are very explicit, here's an example of this song after it has been edited to not include any of the explicit language:
Another factor of the legal side to things, there is a law in this country that dialogue must be at a certain volume for audiences to here. However, some exceptions can be made for the purpose of the film - 'A Quiet Place' is sci-fi film based upon a family who are living during a global apocalypse after an attack from some beings who rely on sound for their attacks, therefore there is little to know speech and dialogue. Here is the trailer which will give you the full idea of how minimal the sound is:
Changing the order of sound:
In film, sound can be used to change a chronological order of things for the purpose of the piece of film. In the Netflix original 'Bird Box', the film was edited to start with the voice of the main character years after the apocalypse had happened, despite the fact that we follow her journey from the beginning of the chaos. The editing team manipulated the sound of this film to clearly highlight the fact that we were going to switch between flashbacks and flash forwards throughout the entire film. This technique also intrigues the audience from the very beginning to the mystery of what is going on in the world and opens their minds to what could be causing the events in the flash forwards. The voice overs from Sandra Bullock's character create the continuity leading onto the intense events of the ending of the film. Here is the opening sequence to 'Bird Box' which will illustrate exactly what the producers wanted to do with it.
Creating impact:
In all movies and television series, you will find that sound effects and music are used to add impact to what is said or done on screen. One of the most common examples i have found personally is in television shows when there is an emotional moment between a couple of characters and mood-setting music is added in the background to create the perfect atmosphere. Here is an example of where this happens in season 3 of MTV's 'Teen Wolf':
The audio track in the background is placed there to create the romantic and magical atmosphere of the moment on screen. This is known as non-diegetic sound as it does not have a source from the world of where it's happening, and to broaden your understanding of this I should point out that diegtic is sound that comes from the world on screen, for example: speech or background noise like car sounds.