I would like to thank mikeford, Sammy1, and pcCinema. What they were suggesting was spot on. Basically, (1) specify in audio control panel to use digital out and (2) specify in media player not to decode but to use SPDIF passthrough.
I have the same motherboard (Intel DG965OT), and I had the problem.
Here's what I did to get sound to all my speakers:
1. Intel Audio Studio | Setup
2. Output Mode -> 5.1 Speakers
3. Digital Ouput Mode -> Dolby Digital Live
4. Set your media player to use SPDIF.
For step4, it will depend on what media player you use. In my case, I used Media Player Classic, so I will explain for it.
Set SPDIF for Media Player Classic:
1. File | Open DVD.
2. While the movie is playing, right click on picture, Filters | AC3 Audio Decoder
3. AC3 Decoder Settings -> SPDIF
4. DTS Decoder Settings -> SPDIF
5. AAC Decoder Settings | Downmix to stereo -> uncheck(Not sure if Step 5 makes any difference, but had it unchecked anyway.)
By default, I had the following:
AC3 Decoder Settings -> Decode to speakers: Stereo
DTS Decoder Settings -> Decode to speakers: Stereo
And that's why I could only hear in the two front speakers, but nothing in the rear speakers. My guess is that you are failing at step "4. Set your media player to use SPDIF." Looks like you are using Windows Media Center. Sorry, I'm not familiar with Media Center audio setup.
For what it's worth, I spend 2 days scratching my head over this, trying many drivers, and pulling out my hair.
Hope this post helps someone.