Many people have had problems getting the sound to work through their HDMI cable when connected into a TV after installing Windows 7. I had the same problem and looked at a multitude of sources including windows forums, Sony’s driver and support pages and found no drivers that worked. I even talked to Sony support and they advised me to downgrade to Vista again so they could troubleshoot with me. Not going to happen Sony!
Ironically, in installing a multitude of drivers I ended up un-installing the SigmaTel High Definition Audio driver and then found upon clicking on the increase sound icon in the tray Windows 7 discovered I had a driver issue problem, or audio driver missing. It then proceeded to install a default Windows 7 audio driver (High definition Audio Driver, Driver version: 6.1.7600.16385) and hey presto now through the sound settings HDMI was no longer greyed out and I was able to set it as default. Now the HDMI cable supports audio and video into the TV.
I hope this helps other people with the same frustrating problem!