Windows 7 includes a new feature that reduces the speaker volume automatically when communications activity is detected. For example, many online games (such as Left 4 Dead) include the ability to use a microphone to talk to team-mates during online play. In this instance, Windows would automatically decrease the speaker volume, resulting in low volume during gaming.
One way to fix this is to disable the automatic communications volume change as follows. To begin, launch the control panel from the start menu:
Then, click Hardware and Sound:
Now click Sound to go to the audio properties page:
Click the Communications tab at the top of this window, and select Do Nothing from the only dropdown box. Once you are done click OK. Next time you play a game, the volume should stay at the usual level when you are using a microphone or headset to communicate: