Hi,
I realised this when typing an inbox message to someone today. If you have the edit field open and alt tab away, no matter what your settings may be, you hear everything- be that game events, chats, both sounds as well as tts come through. It completely overrides the settings that you may have configured.
To reproduce, you can do something simple like this.
Go to the forum, open any thread. Ctrl+R to reply, presss enter to open the edit field. While the field is open, alt tab away and you'll observe that everything is being read (and heard). Close this by pressing escape or enter, and things are back to normal again.
HTH