So, lots of people have been having the same issue, including me in the past. "The default setting of Windows 7 is to reduce volume when incoming communications are detected." This is windows 7 thinking its smart and trying to help you out. It thinks empires is a communication program, therefore it lowers the volume of all other applications (including teamspeak 3). Here is how to disable it: Hope this helps some people.
Done, I had that problem around Vista/7 (I don't remember exactly) times and I only found out after reading some random article. That's wonderful design by Microsoft. (their sound engineer 99.9% doesn't play games)
This may well be the case with vista but I have never used it so I would not know. It's a fine example of a feature which could be helpful but should be disabled by default.
