I’m not sure why this happens, but I solved this trouble in the past when upgrading from 18.04 to 20.04, simply configuring the AlsaMixer or PulseAudio from the Terminal with one of these two solutions:
sudo apt update && sudo apt upgrade -y && sudo apt install -y alsa-utils && alsamixer
(Explanation: First update/upgrade automatically the system, then autoinstall the Alsa utilities and mixer too, in case they don’t are installed on your computer).
Then, verify if something is disabled and enable it again. You need to select first the soundcard or input you want to verify, using the F6 key (if you’re using a laptop with FN keys activated in BIOS, probably you should use FN+F6). After you selected the correct source, then you can use the left/right arrow keys for selecting the channel you want to modify. Then you can in/decrease the volume for each channel using the up/down arrow keys or +/- keys.
If Alsa Mixer doesn’t solve the trouble (or you don’t want to install another mixer and use only the Pulse Audio), then try this… First verify Pulse is properly installed:
sudo apt update && sudo apt upgrade -y && sudo apt install -y pulseaudio pavucontrol
(Explanation: First update/upgrade automatically the system, then autoinstall the PulseAudio and GUI control too, in case they don’t are installed on your computer).
pavucontrol on the Terminal in order to open the App or simply press Super key and search for PulseAudio Volume Control from the Applications Menu. Then you can configure the PulseAudio and repair some troubles you find without using the
killall command, if is not strictly necessary to do this…
Hope that help you!