Right-click on the Volume Icon in the Notification Area. Select Recording Devices then Playback Devices. In that resulting window right-click on the listed devices and also on a blank area, and play with the available options to configure your sound.
If that doesn't solve anything go to Realtek and download the latest Vista driver for your Sound Card.
Other options to consider are using System Restore which will remove the recent updates since the selected restore point, or manually uninstalling the updates, one at a time, until the sound returns.
Are there any clues in the Event Viewer or Device Manager?
Perhaps removing the Realtek sound entry from Device Manager, rebooting, and allowing Windows to rediscover it might help?