What to do:
Drag the master volume up and down. See how the app volume sliders
move in sync.
Click mute. See how the sliders go to zero and how the button
changes to “restore”. Click again to restore OR drag the master up
to restore OR drag any app slider up (to restore only that app). See
how the mute button changes state however you restore.
Drag the master to zero. See how the mute button changes state.
Restore by dragging the master back up or using “restore”.
Switch to the "slider" visuals (tiny radio buttons at the bottom
Drag the application volume sliders up and down. See how the master
volume always adjusts when an app volume would pass it, so that it
always corresponds to the loudest source.