Quicktime Screen Recording Not in Mono

A workaround (for listening):

System Preferences -> Accessibility > Audio > Play stereo audio as mono

Remember to disable it after your specific use to avoid unwanted forced mono listening.

Another solution (for recording) would be to create a virtual audio device splitting the real audio input in Left and Right through a third party app like Loopback or Blackhole.

The listening workaround works only locally. A shared file would still remain with the right channel muted.

The recording via the virtual device would instead produce a real mono file with both channels active (identical), ready to be shared.

Another option – more ”time consuming” – would be to open the recorded file in an audio editor, copy the left channel and paste it to the right one and export the new audio file containing both channels active.