After the completion of the rough cut, the material is duped. The picture editor keeps the original edited workprint and a dupe of the production soundtrack. The sound editor receives a dupe of the edited workprint and the original of the spliced production soundtrack for sound editing. The sound editor performs several tasks to the sound. The dialogue of each main character is separated and spliced onto individual tracks so as to
facilitate the final mixdown.
In fact, all of the sound elements (dialogue, effects, music, narration) are eventually checkerboarded onto separate tracks. This permits the dubbing mixers to establish individual volume levels and equalization for each track, and are thus able to deal precisely with overlaps, fades, special effects, and any changes that occur end to end with each other.
Unwanted ambience occurring on the same track (such as between an actor’s words) are cut out. This editing process is known as "flipping the track", because in 35mm that is literally what they do. Since 35mm has sprocket holes top and bottom, the editor merely has to invert the unwanted section so that the base side goes where the (sound) magnetic emulsion was, and vice versa. In 16mm, they use leader.
Sounds of very short duration are merely erased from the track by mechanically removing some of the magnetic oxide emulsion with a razor blade, sandpaper, or cleaning solvent.
Totally unusable dialogue is replaced with ADR
In the course of dialogue editing, the sound editors will often come across a section that is full of unnecessary splices or contains damaged sprocket holes. Requests will be sent to the Sound Department or lab to have these takes retransferred from the original 1/4-inch tapes. These reprints will then be meticulously spliced in to replace the damaged sections. (This is why all transfers must be done to industry standard on well maintained
equipment - so that reprints intermatch original.)
Sound effects are added wherever necessary, including the creation of ambient
backgrounds. Foley is recorded for the footsteps, body movements, and some sync sound effects.
Narration tracks are laid in, as needed. Checkerboarding is used so that the mixers can correct any audible changes that may occur when different sounding takes are joined end to end.
Finally, music editors will assemble the music tracks, cutting them to match the appropriate picture sections in terms of length, climax, and fade points. As with dialogue and effects, the music tracks are checkerboarded for the mix.