This should reset all the sliders back to the max but leave your master volume where it is. Then click the Reset button at the bottom. Then at the bottom under Advanced sound options click App volume and device preferences. One simple way of doing it in Windows 10 is to right click the volume icon in the system tray and go to Open Sound Settings. You could also set the master volume to max and then drag up all the sliders individually but that also has it’s drawbacks.

You can move them all individually back to the same level as the master volume slider but that is kind of tedious. I wanted to save how to reset the volume mixer in case I ever forget so I decided to post it here. I haven’t figured out why this happens but it sucks. Occasionally for no apparent reason applications in the Volume Mixer are losing their settings and seemingly randomly changed so that some applications are no longer moving with the master volume slider.