OK I had already posted in the Vista forums but didn't get a good answer. After I installed my Visiontek 4850 my sound stopped working. So I went into audio devices to re-enable my speakers, knowing it would default me into the HDMI set up that the cards have. Only to find that I have no other devices showing now, no speakers, no headphone, nothing. I have the box checked that says "show disabled devices" and "show disconnected devices." I had formally been using my onboard sound as well.
More about4850 sound issues
What's happening is your 4850's HDMI is being seen as a soundcard by your BIOS causing it to automatically disable your onboard sound.
Go into your BIOS and see if there is an option to force your onboard sound on.
Use CCC [Catalyst Control Center] and see if there's an option to disable the sound capabilities on the 4850. I have a Nvidia 8800GT card so I'm not sure it'll work.
I couldn't find a spot in the CCC to disable it. Also I am not knowledgeable enough to know what I should do if I tried to fix it in bios. If someone would want to walk me through it that would be great.