Splitting and regrouping stereo's right and left channels (Stereo to Mono)
Using ffmpeg, a free command-line tool,

ffmpeg -i input.mov -c:v copy -af "pan=1c|c0=c0" out.mov


This copies the video over, and the audio is mono with the first channel of the input mapped for export.

Some players may not be setup to default to a stereo matrix. In which case, you can generate a faux stereo stream.

ffmpeg -i input.mov -c:v copy -af "pan=stereo|c0=c0|c1=c0" out.mov