1. Find the name of your audio sink by running

   pacmd list-sinks | grep name:
   

2. Then run this command (taking care to remove the angled brackets):

   pacmd load-module module-remap-sink sink_name=mono master=name_of_audio sink_given_by_previous_command channels=2 channel_map=mono,mono
   

   or add

   load-module module-remap-sink sink_name=mono master=name_of_audio sink_given_by_previous_command channels=2 channel_map=mono,mono
   

   to /etc/pulse/default.pa to have it run at startup.

3. Then in Sound Preferences choose “Mono” as the output, but remember to reduce volumes by half, since two channels are getting mixed into one, or else you’ll have distortion. To test, run:

   speaker-test -c 2 -t sine
   

Same thing in a single command:

pacmd load-module module-remap-sink sink_name=mono master=$(pacmd list-sinks | grep -m 1 -oP 'name:\s<\K.*(?=>)') channels=2 channel_map=mono,mono

4. To remove the mono channel, just use:

   pacmd unload-module module-remap-sink
   

Source: sound – Can I downmix stereo audio to mono? – Ask Ubuntu | https://askubuntu.com/