You are not alone with the problem of drivers for the Sigmatel Audio on Vista. I haver tried at least a dozen fixes from advice given on Google all to no avail.
The only way that I can get the sound to work is to use the generic High Definition Audio Device from Microsoft. Go to Device Manager and Uninstall the Sigmatel driver - make sure that when you uninstall that the remove driver box is ticked. Then go to the top of Device Manager and click on Search for New Hardware. This should install the default driver and I know that this works for Vista Home Premium.
If you find another driver that works please let us know. The only difficulty I have with the Microsoft driver is that it doesn't work on my front panel headphone socket.