I can't get this to work either (in Vista - is this what you're using??).
I have Realtek onboard HD audio and my DVD's ripped to the hard drive play in DD or DTS fine.
As soon as I install FFDShow and/or Haali Media Splitter (for .mkv playback) all my DD & DTS soundtracks are downmixed to 2ch stereo as well as the divx/xvid/mkv files. It's so bloody annoying.
In the Audio control panel, you can click the test buttons for the various surround options and it plays it back perfectly, but any content with multi channel audio is rendered in stereo. It's a common problem too, I've discovered in my Google searches.
I'll give Vista another week of tinkering and if it doesn't work I'll be plugging in my old but working perfectly MCE2005 machine.
D.
PS - sorry I'm not much help!!