With this plugin, you can easily control your Game Sound Volume once it loses focus.

When the player alt-tabbed to a different application, or actively switches to the second monitor, all the audio from your game will remain.

Download and try this little Demo.

There are two ways you can use this functionality: simple - toggling with a bool 0/100%, or adjust to some preferable number with the float, e.g. 33%, making your game volume dim, so the player will still be able to hear it.