In the Windows 10 Fall 2017 Creator Update Microsoft had updated the audio mixer to monitor and control any apps using the sound system.  Now, instead of having to navigate to each app's sound level controls, you can raise, lower or mute any one app that's actively playing sounds.

This allows you to easily find a volume setting and change it without bringing the app to the foreground and clicking on its controls.  In some apps (MS Movies and TV for example), there is no obvious control especially when it is in borderless mode.