I also had this issue, 21.2 Cinnamon, and it happened after installing the Displaylink drivers. I thought I was going crazy missing sound and audio.
My issue was that the pulseaudio.service was removed from user startup. If you wanted to verify this is the issue, in a terminal, you can send a
That "kills" and restarts the process.
If it complains there's no process running, just sending this will start the process:
You should then see your pulseaudio icon in the tray, and sound would be restored. But the process is running in a terminal, so closing it would end the pulseaudio process.
To add to startup permanently:
Open Session and Startup (from Mint Menu)
Choose the Application Autostart Tab
Click the + sign at the bottom Left
Name: Start PulseAudio
Description: Start PulseAudio Service
Command: systemctl --user start pulseaudio.service
Trigger: on login