Ok, I had been having the same problems as it seems everyone else did. I
tried multiple drivers from Realtek.tw, (Ac'97 and HD drivers) but I also had
the red cross over the icon. If I double clicked it, a box told me there were
no devices installed, but if I check the properties, the audio stuff was
installed.
One interesting side note before I give the solution that finally worked. If
the computer went into sleep mode, the sound would work when I woke it up
(and green sound lines appeared instead of the red X).
It works now, and there are only two things that I did. I uninstalled the
EZtrust antivirus (not sure if this affects it or not). Then I uninstalled
the driver for the sound from device manager. When I restarted the computer
Vista recognized the sound device and found a driver somewhere (don't know
where) that works now.
I hope this helps.