I looked at Soundsource and BackgroundMusic, and neither of them have hotkeys to increase or lower the volume of the current app. Ideally I want to and see the volume level like you do when pressing the volume up and down keys.
Roblox for instance, doesn't have hotkeys for changing volume. You have to press ESC, click settings, scroll down to volume, then click 1 of 10 buttons in the "slider".
On soundsource, you have to open the menu and go down to the app you want to change the volume of.