That would be the optical soundtrack from the print, or optical track, containing the Dolby Stereo 2-track mix. The audio has some subtle background pops and scratches but it doesn’t overpower the sound mix. These anomalies are what audiences in the theatre watching the film would have heard at the time of the showings. The optical track audio would be in sync with the video because it came from the print. That audio track should have came with ISO file. On the spleen, that audio is the only audio on the ISO file.