To compress and summarise a sound recording to fit a specific space
Another famous example would be the Friends intro of the hit TV sitcom. The intro is just a 44 second snippet of the 3 minute song "I'll be there for you" which has been cut down to fit a space of 45 seconds in order to match up with the quick snappy compilation of clips taken from the show itself. This again, has since become a staple for TV show intros and is again, fast paced entertaining for audiences.
To eliminate flawed, superfluous, repetitive, uninteresting, irrelevant material from the clip
To enable recorded speech to remain legal
This is to deter from copyright infringement and invasion of privacy. This is used to check any background audio that wasn't permitted to use or may be illegal. This may include speech from strangers picked up in the background. While this interferes with whats happening on screen, it is also illegal to record someones conversation without their full permission otherwise legal action can be pursued unless the audio is deleted or cut out. Another example would be background music picked up at a location. If the rights for the certain song is not obtained then the creator/artist may also pursue legal action as they do not have the rights to the property. These illegal uses of audio recordings can lead to lawsuits or re-shoots which may cost a studio a lot of money so extra precautions are taken to edit our or prevent illegal speech to be recorded.
To ensure any inappropriate or indecent language is edited out
To change the chronological or logical audio of a series of sounds or recorded speech
The most notable example of this would be in film trailers where both visuals and audio would be switched around and compressed in order to confuse an audience by making the narrative different or unclear. It is used to attract and audience to a film by leaving them with mysteries so they come and watch the finished product in order to gain clarity. An example would be the Kingsman 2 trailer which shows multiple action sequences in different orders. During this the song being played is cut and reordered so that upbeat parts of the song match the fast pace action of the trailer. Lyrics of the song may be faded out to shift the focus onto the dialogue of the characters.
To create impact by adding sound effects or music between pieces of dialogue
Sound effects are artificially created and added into films to emphasise or express and action, mood or feeling. They would be synchronized to certain actions. They are also used to create ambiance in order to create an atmosphere or to build anticipation.Sound effects are recorded as Foley sounds or are created using digital software. These sound can be played close together in order to create realism as sounds are usually concurrent in real life. Sound also may be layered in order to create new sounds entirely. This practice is most common in Sci-fi films to express new environments or creatures to entice an audience. A common use of sound effects to create atmosphere would be noticeably horror films. On the other hand, music can be added in order to display the emotion of a film or to enhance it from a story telling perspective. The strongest form of music in film is that used to create an atmosphere by matching the tone of the music to the mood of the film. This means that the music audio would match what is happening on scene. Music can also be matched geographically with a film in order to establish a location. For example a beach scene would have happy music to match the bright colors and tone on screen. Where as a haunted mansion would have very eerie and ominous audio played.