Hey,
If you're using Windows 7, you may want to disable the auto volume messer thingy on it. It tends to mess with volumes if you have games/voice com software running.
You can disable this by doing the following:
1. Click the volume icon in the notification area and when the volume slider appears, click "Mixer".
2. When the main Volume Mixer appears (showing custom volumes for apps and stuff) click the "System Sounds" icon. Then the "Sounds" dialog should pop up.
3. Finally, when the "Sounds" dialog shows, goto the "Communitions" tab, and from the four options, select "Do Nothing". Like the screenshot below...
Let me know if this works