Sound effects can be so subtle that they are barely noticed, but they add complex layers to the sound of a film. While some of the production sound is left in, such as car doors slamming or windows smashing, other sounds must be added. In this case, sound effects may come from an existing sound library or be recorded especially for the movie from the source itself, such as the engine noise of a sports car or the thundering hoof beats of a racing thoroughbred.

Other sounds are created on a stage through the process known as Foley. Foley is used to make new sounds or enhance sound effects that already exist. Foley artists use props to record sounds in sync with the picture. It’s not uncommon to see a Foley artist walking in place on linoleum to create the sound of footsteps or hitting a raw steak to imitate the smack of a punch.

Music adds an important element to a film’s sound, whether it’s an original score or source music, such as a popular song or a sequence from a symphony. The director watches the film with a music editor and composer in a music "spotting" session to select places in the film where a music cue makes sense. The music supervisor or music editor then selects source music for the director to listen to, and the film composer writes the score.