Answer 2 arc22

Right, Globvler doesnt mix sound output from different apps. So you cant mute one app while listening others.
You still have one volume control. It changes while youre switching from app to app.
I.e, you can select maximum volume for Screensaver and Minimum for Pages...
App does only change global volume, nothing more.

You need another app for this task. And youre not alone who want this.
In such case, that app will require a kernel extension or Core Audio plugins and root permissions to be installed...
It isnt allowed by current MAS rules.

Alexey Galygin about Globvler