I was using Windows version 1909 before this and never had the system sounds change its volume. I've already googled quite a bit and tried most setting changes to no avail. He was even convinced that this was a feature of clicking on the slider button, which seems a bit odd to me. I checked with a friend via Discord screenshare to see if the same would happen on their end and it did. Click the slider button (so it pings with an alert sound).Open the volume slider by clicking on the speaker icon in system tray.
I can recreate this bug most of the time by doing the following:
Whenever I adjust the 'System Sounds' in the volume mixer and then close the volume mixer, soon it will reset back to 100% and it will even unmute itself. The only applications I have installed at the moment are Chrome and Discord.
I just did a clean install of Windows 10 20H2 on a new disk and applied the latest updates and drivers.