Nah, you're not missing anything as I've tried searching for it too! I like the idea of just having an option to mute sound while tabbed out though, it's more convenient
not saying this will be a solution just a way to do similar, right click your volume icon > open volume panel ( translated lol ) and just mute the sound for soulworker? should be easy and fast if you just want it to mute while tabed out 
I am doing this. But it requires me to do three steps. It becomes annoying and inconvenient really fast if done repeatedly.
you should be able to take it down to two, if you leave the panel open hehe
well should be simple to add hotkey for muting, do hope they do a simple update like this for you and probly more that want this
I've seen it some other MMORPGS like Guild Wars 2.
There is a option you can tick... This will auto-mute the game when alt-tabbing out.
Please add!
