Interestingly, the film's "audio" was essentially "doubled" during production—Hanks recorded clean lines in a studio, which were then played back at Skywalker Ranch to capture the natural acoustics of an open environment.
He writes:
A dual audio file contains embedded within a single video file (usually an .mkv or .mp4 ). This allows you to toggle between the original English dialogue and a dubbed version without needing two different files. Why Dual Audio is Unique for "Cast Away" cast away dual audio