HDMI sound not working after upgrading to 20.04

Reboot should not be necessary if you just need to reinitialize pulse, just do a:

killall pulseaudio; rm -rf ~/.config/pulse/* ; rm -rf ~/.pulse*

and you should be good as well.

Suggestion of Raffa works for me on a laptop Asus UX303L !

rm -r ~/.config/pulse/
reboot

and it's ok now

In my case, following upgrade to Ubuntu 20.04, HDMI output is just not listed anymore in sound settings if I plug or turn on my TV after logging in.

I am not sure this will help with the particular issue you are encountering, but the workaround for me is to open a terminal and run:

pulseaudio -k

then re-open the sound settings, where HDMI output can be selected and works as expected.