When you mean no sound, do you get the volume slider but nothing comes out or do you not get the volume slider at all?
Go to control panel, sound (you might need to switch to classic view to see the one im on about), then the "sounds" tab, there should be audio clips that you should be able to play, if the laptop has picked up the sound controller in it.
If you cant play these (greyed out) then the driver isnt installed properly.
There should be a update option in the boot camp program that is installed in your vista partition, run that, and if it fails you can try what is below.
Try going into device manager, uninstall/remove the drivers under the heading "sound, game and audio controllers" reboot the machine and it should install them back in again.
If it doesnt and asks for a disc, then they'll be on your max os x disc, but not sure you can install drivers individually this way. might have to reinstall the boot camp drivers from scratch.
Ive had this proplem on PC's (not macs) with drivers from manufacturer of the machines not working after windows updates. Fixed by using drivers from the audio controller manufacturer. But dont think its the same issue you are having.