No audio sound after a system upgrade (sudo apt full-upgrade).
Since the new release of Raspberry Pi OS from 4th of December 2020, which switches to use the PulseAudio sound server. In some cases, this might affect the audio output settings after the system upgrade.
To correct the problem, see the instructions HERE under the sections of How do I get it? and Updates.
Or follow the steps below:
1.Update your system to the latest version
sudo apt update && sudo apt full-upgrade
(It is safe to just accept the default answer to any questions you are asked during the update procedure.)
2.Install the PulseAudio Bluetooth support
sudo apt purge bluealsa sudo apt install pulseaudio-module-bluetooth reboot
3.Swap over the volume and input selector on the taskbar from ALSA to PulseAudio, after your system has restarted.
a) Right-click a blank area on the taskbar and choose Add / Remove Panel Items.
b) Find the plugin labeled Volume Control (ALSA/BT) in the list, select it and click Remove.
c) Click the Add button, find the plugin labeled Volume Control (PulseAudio) and click Add.
d) Use the Up or Down button to move the new Volume Control to your desired position.
e) Click the Close button, to close the Panel Preferences window.
4.Right-click on the Volume Control icon on the taskbar, select Audio Outputs and choose HDMI if it wasn't ticked.
5.Finally, to prevent some applications that are ignoring the effect of the PulseAudio output switcher, which probably caused by an old ALSA configuration file still being on the system. Remove it with the following command in a terminal.