I guess it's something to do with Vista resetting (pre-)configured default values (had something similar happen to my sound settings once).

I suggest you right click your desktop background (which brings up the Control Panel\Personalization window), click the "Sounds" option (the 'big list'); check (& modify) the options & settings of your devices under "Playback". (Just click the icon of the device in the list that appears, and click properties.) Pay particular notice to "Exclusive Mode" under "Advanced", make sure both options are checked. AFAIK RTW tries to influence sound-ouput directly, so it would file under "Exclusive Mode".

Of course, especially if you're using an external audio-system (not buit-in or monitor speakers); it would probably be worth the effort to check that input settings are configured properly & everything works. For all I know it could be as simple as an audio cable that hasn't been properly attached.