Windows 7 includes a new feature that automatically reduces the speaker volume when any communications activity is detected. Some online games include the ability to use a microphone to talk to team-mates during online play. In this case, Windows 7 automatically decreases the speaker volume.
Low Volume feature in Windows 7
However if you don’t like this feature, you can disable this Low Volume feature in Games of Windows 7 as follows:
Open Control Panel > Hardware and Sound > Sound > Audio properties page > Communications tab > select Do Nothing from the dropdown box > Apply > OK.
That’s it. You will have disabled this feature.