If you've started noticing that your sound is disabled in Windows Vista after you wake your computer from Sleep mode or hibernate, then you are in luck, because Microsoft has a hotfix for this issue.
If you are experiencing this problem, you'll probably notice a red X on the volume icon in the system tray, and a message when you hover over it that says "No Audio Output Device is installed".
The fix for this issue is to install a hotfix from Microsoft, available at the following link:
937077 - A high definition audio device may no longer work after you resume Windows Vista from hibernation or from sleep