Roll back the vista built in HD sound driver. This is a known issue. DELL has not addressed it as of yet.

I had the same problem.. all i did was uninstalled the driver then restarted the computer to see if vista would install a driver for it... in my case it didnt :)

 

but go to this link http://support.euro.dell.com/support/downloads/index.aspx?c=uk&l=en&s=gen

 

enter in your exact model number and download the audio driver install it restart the computer.. then it should work.

 

otherwise youll have to call DELL : /