The-long-and-the-short of it is that when the film was released on May 21st, 1980, it was a limited engagement that was shown only in 70mm prints. When the film went into general release with 35mm prints on the following June 18th, it was new a cut that included changes (some optical-effects fixes and dialogue replacements, among them) and 3 new special effects shots that increased that length of the film by 20 seconds.
Discussion of that limited-engagement cut can be found here.
Someone did a recreation of the ending of that limited-engagement cut on YouTube.
The audio in that video is a vintage In-Theater recording of a screening from that alternate cut.
Details about that audio recording can be found here: