I'm currently using Windows 7 with the latest bootcamp drivers, however when I use HDMI with the Thunderbolt port, I do not get surround sound. Only 2 channels, Windows is using a Generic Microsoft audio driver.
If I use the optical port it works with surround sound, but I do not want to use that port because that means extra cables. So what I need is someone to show me the driver needed or at least tell me the hardware used for the thunderbolt port for Audio. This should be included in the Boot Camp driver package >:(
Download these drivers from AMD for your Radeon graphics adapter. That will get your audio out on the HDMI working with surround. Good luck!