Lately I have been seeing quite a few posts of people asking why their sounds are lowered while running a source based game. Many people assume that the Source engine is responsible, but it is not. Windows is the culprit.
While this knowledge is fairly easy to find on Google, I figured I'd make a quick tutorial here so people can continue to be lazy. (I couldn't find anything about this on MODDB) ;P
Please keep in mind this was written for people using Windows 7 and Vista, I don't think the problem exists on XP.
1.) Open your control panel
2.) Click Hardware and Sound
3.) Click on Sound
4.) Click the "Communications" tab
5.) Under "When Windows detects communications activity:" select "Do nothing"
6.) Click OK
Now all your sounds (Ventrilo, Skype, whatever) should remain the same when you launch a source game.
There you go, simple right? I hope it helps.