Manjaro provides manjaro-pulse and manjaro-pipewire so user can choose either software audio server
There is such a command - pulseaudio — Arch manual pages but it is not needed to remap a PulseAudio sink to mono
Method to force mono audio? - #2 by nikgnomic
Permanently enable mono audio output - #4 by nikgnomic
PulseAudio/Examples/Remap stereo to mono - ArchWiki
That is probably correct if pulseaudio is not found
pipewire-pulse can use PulseAudio module-remap-sink to remap audio channels to mono
pactl load-module module-remap-sink sink_name=mono master=NAME_OF_AUDIO_SINK channels=2 channel_map=mono,mono
Or install easyeffects and set Stereo Tool plugin to ‘Mono Sum L+R’
AskUbuntu - How can I downmix stereo audio output to mono in PipeWire
For pipewire-jack (“pro-audio” profile) audio channels can be remapped manually in patchbay GUI (qpwgraph, helvum etc)
Archwiki does not have such information for PipeWire