I have a brand new Sony Vaio VGN-FZ140E, same problem. Had upgraded to Vista Ultimate from Vista Home. This problem appears to happen when you install Vista or upgrade to another version of Vista. I found the following suggestion on another tech forum, followed it & fixed problem, took less then 5 minutes. You will need to first check the name of the audio device on your computer, write it down before you uninstall it, then go to HP website and find the "original" software. To be safe, better check if HP's website is as user friendly/sophisticated as Sony's and has your audio device's software on it to download before you go and uninstall it. Anyway, here's the solution from the other guy that worked for me:
"Hey, I had the same problem with the audio as it kept saying "no audio ouput device installed."
First uninstall the sound drive by going to device manager and click on sounds. Then click the device tab and click uninstall. Then restart your computer.
Then go to Sony eSupport - Electronics and click on software and ers. Then type in your model number and on the next page click "original". Then look for "SigmaTel
I recently purchase a HP Pavillion a6040n Desktop PC. Everything was working just find until one day, I turn my computer on and the specker icon would have a red icon and it would say "no audio output device is installed". Under device manager is not showing Sound and video, etc. I have checked "services" and the sevice start automatic and is not stopped. I reinstalled the driver from realtek(realtek high definition audio)several times and still have no audio.
I called HP help desk, but it seems I have more knowledge than thier computer techs.