Sound Auto Decreasing when I play TF2

Whenever I play tf2, and I get into a server, my volume for TF2 + Ventrilo automatically decrease and cap to around 10-20%. As soon as I close out of game, Ventrilo volume goes back up to 100%.

I'm using USB audio headset.

Seems I found a fix. For some reason it thought that When I played TF2 it needed to reduce my sound due to placing or receiving telephone calls. Even though I wasn't doing that. To fix it, go into your sound -> communication tab and select "Do Nothing" from the drop down menu.