In order to permanently set the audio output of the VLC
media player to "Mono" by default, edit
the equivalent) and set the
[core] → stereo-mode line to
stereo-mode=7. Why 7? Because!
Thanks to #videolan on Freenode for the tip.
Note that the “Audio” menu setting does not persist by design.
Copyright by Shlomi Fish, 2019.