I had the same problem when I up graded from Windows XP to Windows Vista Ultimate.
When I was using XP I had no problems with the sound on my PC but as soon as I loaded the disk for Vista Ultimate I got the message No Media Devices are Installed when I placed my pointer over the speaker with the red cross on it over on the right hand side of the screen, next to the time display.
I had this problem for 2 days and was looking on line for ways to try and resolve it but was un sucessful.
I looked on my Controll Panel and double clicked on the System location and went to the Device Manager.
Once in Device Manager I noticed on the other devices section it was showing a Multimedia device but when I double clicked on it and tried to install and update driver I was receiving a message telling me that no drivers could be found for the device.
I then thought about other things I had had problems with in the past that did exactly the same in the devices section and thought to myself, well all I had to do to resolve those problems was un plug the device from my mother board and plug them back in after a few seconds and the computer downloaded the drivers for them straight away.
I then thought of what audio devices I had pluged in to my mother board and realised that the only things to do with audio were the speakers.
I thought to myself, it may not work but I'll give it a try so I un pluged the speakers (BOTH WHIRES) left it for a couple of seconds and pluged them back in and HEY PRESTO, I now have full sound with no problems at all.
If your PC came with a built in sound card and you are having the same or a similar problem, I'm not saying it will work but give it a try to see what hapens.
As they say, you'll never know untill you try.