Any WMV-HD you purchase will include WMA Pro 5.1 audio and not Dolby Digital (DD) audio. While many can argue about the sound quality of the two, the main problem is that most people are not going to pay $4500 to get a new AVR just to decode WMA Pro 5.1. So the well known AC3 Filter (Open Source) will give you a workaround for this. I think it’s up to RC5 and more information can be seen below. I tried this with my HP and SPDIF out and it works like a charm.
Doom9 Thread: http://forum.doom9.org/showthread.php?s=&threadid=76480