Find the name of the output sink for headphones/speakers
pactl list short sinks
For an onboard audio device, the name will look similar to this:
alsa_output.pci-0000_04_06.0.analog-stereo
Use this command to create the additional configuration for PulseAudio in user folder
using the correct name for the (master=) output sink
and set the remap module to be the default sink for audio playback
cat <<EOT > ~/.config/pulse/default.pa
.include /etc/pulse/default.pa
load-module module-remap-sink master=alsa_output.pci-0000_04_06.0.analog-stereo sink_name=mono sink_properties="device.description='Mono'" channels=2 channel_map=mono,mono
set-default-sink mono
EOT
Restart PulseAudio to load the additional configuration
systemctl --user restart pulseaudio
[ Archwiki - Pulseaudio Examples - Remap stereo to mono ]
There is no need to copy this file to home folder: All options in /etc/pulse/daemon.conf are inactive
Default values are commented out. Use either ; or # for commenting.
Changing default-channel-map might work ok instead of using the remap module, but I am not sure if it will work with the equalizer
Default setting default-sample-channels = 2 does not need changing
I suggest try this command to change default channel map
tee ~/.config/pulse/daemon.conf <<< 'default-channel-map = mono,mono'
Use this command to check daemon.conf configuration
pulseaudio --dump-conf