I think this is likely to be due to out of phase audio in one of the stereo channels. Below is a copy of my answer from here: Premiere Pro audio muffled and distorted on export
(I have tried to flag this as duplicate, but cannot because the answer wasn't accepted).
This might be because you are exporting a stereo audio file which has one channel phase ...
I want to cut an existing movie and want to remove all the language track from it.
ffmpeg -i -c copy -an video_only.mp4
How do I remove all the soundtracks except the one I need?
ffmpeg -i -c copy -map 0:a:1 specified.mp4