I had a similar problem with loss of sound on my Sony Vaio laptolp. I use Vista home premium and recently purchased driver scan on line. It flagged several drivers that were out of date. I diligently downloaded the newer versions - which included updates for my sound drivers.
Once installed and on reboot I lost my sound!
In the end I resorted to selecting a previous restore point and it brought my sound back. Now....have binned driver scan...dont trust it anymore!
My sound drivers are out of date I guess but hey....better than no sound at all