How do I make Windows mute background applications?

Solution 1:

For some reason this feature seems impossible in Windows. I've been researching on google, you and me seem to be some of the only people who want this feature.

HOWEVER I HAVE A SOLUTION FOR YOU! On Steam (and likely elsewhere) there is a utility called Borderless Gaming. Its main function is to allow running a fullscreen game as borderless windowed (for games that don't support this.)

However it also has an auxiliary function: "Mute In Background." And it does exactly that. You can add your games to its interface, and tell it not to mess with your window size. Just mute when in background. And it works.