Quicktime Video Panning multiple inputs problem
I don't think Quicktime Player is capable of this, but you could run it through ffmpeg afterward to change the audio to mono.

ffmpeg -i stereo.flac -ac 1 mono.flac

If there's video too then:

ffmpeg -i recording.mp4 -map 0 -c copy -ac 1 mono.mp3

-map 0 copy every channel from the first input (the only input in this case) -c copy copy the same codec used for encoding for speed and to avoid quality loss, -ac 1 set the output to only have one mono channel. (The extensions are interchangeable.)

See:
https://trac.ffmpeg.org/wiki/AudioChannelManipulation