Vista has an known issue where, after hibernating your computer or putting it to sleep, the audio no longer works when your computer wakes up. You can tell if you are experiencing this known problem because you'll see an X in a red circle next to the audio/volume icon in your system tray and when you hover over the icon you'll get a message that says there is No Audio Output Device is installed. If this sounds like you, the good news is that Microsoft has a fix, you can find it here: http://support.microsoft.com/kb/937077/