Is this a desktop or laptop? Do you have your sound on a seperate card installed on the computer or is it attatched to the motherboard? usually if it is attatched to the motherboard the sound ports will be beside your monitor, keyboard, mouse port. make sure if the sound is a seperate card that it is pushed all the way into the slot into the motherboard. If it not fully seated this will also cause this problem.
you can do a manual scan for hardware changes in device manager by going to Computer-right click, select manage, then right click sound and select scan for hardware changes.
I am not sure but it could be your sound device driver. you may need to install the device driver for the sound card. This is a disk that come with the card or with the computer if the sound card was already an installed component. If you can't find the disk you can go to the Sound card manufacturers web site and look for the device driver and download it. After the download is complete you will need to do a restart and your sound should then work. After the reinstallation of Vista you need to install all additional drivers for other hardware. The Operating System (Vista)holds many drivers itself wich is how other components are working such as your mouse, monitor, video card,etc. but not always, which is why you need to go to the manufacturers web site and download it. I hope this helps.