in case you have a wonky mic where the left channel sounds much louder than the right; here's an ffmpeg command to keep only the left channel in a video file, converting it from stereo to mono:
ffmpeg -i in.mp4 -af "pan=mono| c0=FL" -c:v copy out.mp4
alternatively, EasyEffects makes it simple to set up the stereo→mono mix once, then have it applied on every boot: https://github.com/wwmm/easyeffects (has nifty effects, too)