Ran a update today (4-15-2021) on my Ubuntu 20.04.1. The update asked for a reset and I think I saw something about kernel 5.4 before the update started (after the update my kernel is 5.4.0-72-generic according to the uname -r command).
After the reboot I had no audio (I use the HDMI Output to my monitors built-in speakers). I checked my Sound Setting as some kernel updates in the past have flipped my sound output to Digital Output Built-in Sound. I checked and sure enough it was flipped. When I went to change it back there was only one sound output option (Digital Output Built-in), no HDMI to be found. After some Googling I found this solution that worked for me:
Step 1
Open a terminal and run the following command:
pulseaudio -k
This kills the running sound process
Step 2
Physically unconnect then reconnect your monitor from the HDMI port while Ubuntu is running.
Step 3
Open Sound Settings and you should see the option of HDMI in the Output tab:
