Half Life 2 sound annoyance

[minerva000]

Okay whenever I run Zombie Panic Source or any Half life 2 game it muffles the other sounds of things like Pandora or a my Windows media player..

Zombie Panic Source is a Half-Life 2 mod.

(Muffle being lowering the sound volume significantly and it will not be restored to normal until I close the application)

Can anyone tell me how I can get it to stop doing that, the sound options within these games are fairly simple and clear and I've tried changing some settings but I don't think these are the problem.

Thanks for your time.

[Jaytee]

I had problems with HL2 if I ran other programs but found it fine if it got 100% of the computer.