If the video was created with multiple audio tracks this effect is quite possible.
ffmpeg can be used to reencode it with a single audio track.
Without knowing the original encoding it still is relatively simple to pick out the first audio track for a copy.
Code:
ffmpeg -i original.mp4 -c:a:0 copy -c:v copy outputfile.mp4
Use 'man ffmpeg' to find out details of how ffmpeg can be used and to understand what each of those options actually does.
Ffmpeg is even able to change the number of audio channels in an audio track. I have reduced a 7.1 track to both 5.1 and to stereo when needed.
