Better OSD is what you're looking for. It works very well on Ubuntu 18.04.
It is a Gnome extension, that is available via Ubuntu Software too.
After installing it via Ubuntu software, you can access to its settings via Gnome tweaks>Extensions (if you don't have Gnome Tweaks, you can install it from Ubuntu Software too).
Here are some screenshots of what I ...
Alternatively, system-wide solution using pulse:
Get sink list
pactl list sinks | grep name:
Use remap module to map channels
pactl load-module module-remap-sink sink_name=mono \
master=<sink_from_prev_cmd> channels=2 channel_map=mono,mono
To stop it, Unload the module
pactl unload-module module-remap-sink
For permanent setup, add command from ...