https://www.winhelponline.com/blog/taskbar-volume-control-not-working-windows-10-sndvol/
Fix 1: Restart Explorer via Task Manager
The volume control icon may fail to respond intermittently for unknown reasons. The quickest way to resolve the issue is to restart the Explorer shell.
- Right-click on the Taskbar, and click Task Manager
- In Task Manager Processes tab, scroll down to Windows processes category.
- Right-click Windows Explorer in the list, and click Restart
(For more information, check out the article How to Exit Explorer and Restart Explorer in Windows 10)
This restarts the explorer shell and refreshes the Start menu and taskbar (Shell experience host). See if the volume control button works now.