After installing the vista SP1 update you might notice that there is no sound and the speaker icon on the system tray displays the following message No Audio Output Device is installed ,what seems to work is to remove the audio driver from Device Manager.
To do this click Start ,Control Panel ,System and Maintenance and then Device Manager on the left pane.
On the Device Manager dialogue box click on the Hardware tab and then Device Manager ,expand the Sound ,video and game controllers section ,right click on the current sound device(Realtek ,Via ,soundblaster etc.)and choose uninstall ,do not check the option Delete the Driver Software for this device unless you have the installation CD or software for your sound device.
After doing this restart the computer and Vista will reinstall the device and the sound will work again.