Viblyy for Windows.
The same quiet for music that plays in an app — the Spotify app, Tidal, foobar2000, VLC.
ViblyyPlaying
V
VLCpaused by you — left alone
A sketch, not a screenshot.
Where it stands
Half of it works. It can turn one app down and back up without touching the rest, hold the level when a player tries to put its own volume back, and pause one that refuses to stay down — then take all of it back.
The other half is knowing when an answer has landed. In the browser, that is what the Chrome extension is for. Outside the browser there is no answer to that question yet, and until there is, there is nothing to ship.
Until then
If you listen in the browser — including Spotify installed from open.spotify.com as an app — the Chrome extension already covers it.