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 cases, Windows 7 automatically decreases the speaker volume.
Howvever if you dont like it you can disable this feature as follows:
Control Panel > Hardware and Sound > Sound > Audio properties page > Communications tab > select Do Nothing from the dropdown box > Apply > OK.
Thats it. You will have disabled this feature.