dxdiag has sound check features. You can try to run it and check if there are any error reports, look if no muting is active anywhere in the settings of win7
then enable the windows audio service as you said the sound driver is installed.... probably something that needs to be included into the service pack once
Maybe there are a few more services for that. Its not working meanwhile or?Try to deavtivate and activate again. Maybe that solves it. Not unusual for Windows^^