In larger markets such as New York and Los Angeles, sound editors often specialize in only one of these areas; thus a show will have separate dialogue, effects, and music editors. In smaller markets, sound editors are expected to know how to handle it all, often crossing over into the mixing realm as well.

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Note added at 13 mins (2015-06-23 23:14:29 GMT)
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The sound editor is responsible for everything you hear during a movie. The job is sometimes called sound design because the editor is essentially creating an “aural landscape” from scratch. It’s similar to how a composer writes a symphony. The sound editor must select and balance audio from hundreds of different sources to create a desired effect, anywhere from strict realism to an otherworldly fantasy.