The advent of Windows 7 heralded the arrival of a new OS that made me seriously consider upgrading from XP. The ease of Driver installation under Win7 is wonderous, it just happens! I've upgraded a PC's Motherboard and CPU, Windows 7 just dealt with it - brillent.
I bet that all sounds pretty idiolistic, or worse I work for Microsoft. But alas that bubble has now burst, and no I don't.
A little background first, before a little rant 
Our HTPC was working perfectly on the sound front. The PC is connected to a Yamaha RX-411 AV Receiver via HDMI and the AV splits out the Video from the Sound, processes the sound and spits it out to the 8 (7.1) speakers attached, and the video is passed through to the TV. Playing a file with stereo sound got stereo sound, playing a file with DTS got DTS processing by the AV...etc etc. This was all while running Windows 7 64 RC. Happy users 
Now we've upgraded? to Windows 7 proper and now everything is stereo no matter the sound source format. Its surprising how distressing it is to see KMP showing the movie its' playing is DTS, or 5.1, and the AV has turned off surround processing and is doing the stereo output thing 
and it just sounds wrong!
Install the correct drivers; is the initial response I bet most reading this will have, it was mine. Well that's a hell of a lot easier said than done under Windows 7. From my recent experience, it appears to me that it is no longer possible to over-ride a Windows installed driver with one you think is better, especially if its older than the installed driver.
The PC has an Gigabyte HD4650 video card with HDMI port. Now it appears that the ATI drivers, 9.12 currently, do not install an HDMI Audio driver for this card under Windows 7, the driver my PC is using is a Microsoft Driver of Windows 7's choosing, and this driver does not appear to support anything other than 2 channel (stereo) sound and thus not HDCP.
Ok, the above statement is not entirely true...it seems that the problem revolves around the Auto Install feature policy of Windows 7. The reason the ATI HDMI Driver won't install is due to the Microsoft driver being auto installed first and then Windows 7 blocking the ATI driver; I presume as its dated as older. The solution was a quick trip into the Group Policy editor (gpedit.msc) and effectively disabling the Auto Install feature. Uninstall the drivers and reboot. Then its simply a matter of re-enabling the Auto Install feature, nothing will install while its disabled, and running the ATI driver package - hey presto ATI HDMI Driver is installed and working.
So, after some fiddling with AC3Filter, we're now happily watching, and listening, to movies and things with all the audio channels they have 