No sound when talking in voice chat

[volenal]

Hi! I've been trying to find a solution via google and also this forums search function, but I can not seem to find something that fits my problem.
So my boyfriend is trying to stream a game to me and since teamviewer has almost no delay we've been using that. The only problem is that once I start talking over teamviewer, the game/system sounds of him mute. I can still hear him perfectly fine, but not the game.. Can I turn that off somehow, I've been looking through the settings but I can't find anything that solves my problem
We're both using Teamviewer 11 (the latest version I guess since we just updated a few days ago), he is on Windows 8 and I am on Windows 7.