Control the per-app volume by scrolling over taskbar buttons on Windows 11.
Scrolling over a taskbar button will adjust the volume of that specific application. Ctrl+clicking on a taskbar button will toggle mute for that app. A tooltip shows the current volume percentage, or "No audio session" if the app has no active audio.
Note: Some laptop touchpads might not support scrolling over the taskbar. A workaround is to use the "pinch to zoom" gesture. For details, check out a relevant issue.

For controlling the overall system volume, check out the Taskbar Volume Control mod. Note that both mods can be used simultaneously using one of these approaches:
Note that if both mods are configured to act simultaneously, the Taskbar Volume Control mod takes precedence due to the way the mod works.