If after installing Vista SP1 you find that you have no audio, here is what you may have to do:
Open Control Panel > Sound > Under Playback and Recording tabs, Select and Set Defaults.
Additionally or alternatively, you may also have to try this: Click Start > Type Device Manager in Vista’s Start Menu Search Bar > Hit Enter. Device Manager will open. Expand Sound, Video and Game Controllers.
Locate your audio device. Double click on it to open its Properties box. See whether the driver is installed and the device working properly. In the Driver tab, click on Update Driver. Click OK. Else you may have to uninstall the Driver (but DONT delete it) and then in the Device Manager > Action > Scan for hardware changes. This will reinstall the driver.
For more tips if you continue to experience problems with sound after you install Windows Vista SP1 visit KB948481.