How to force Windows 7 to output 5.1 Surround?

I'm running Windows 7 SP1 x64 on an ASUS M2N4-SLI, which has a coaxial digital audio output (see my profile for more HW info - it's system PC1).

How do I force Windows 7 to output 5.1 Surround to my receiver? When I go to the Properties for that sound device, and I select to test Dolby Digital, I get surround output, but after completion it switches to stereo. How can I get 5.1 Surround 24/7?

EDIT: Here's a snapshot of my audio settings:

It seems that it's being forced into 2 channel mode by something, but I don't know what. I'm open to any suggestions.

As for those of you who would suggest using the 3.5mm jacks, unfortunately my 5.1 amp doesn't have RCA or 3.5mm inputs for use with surround sound. The only inputs that accept surround sound are the coaxial digital and the optical digital. And, of the two, my mobo only has the coaxial digital. Also, the 3.5mm jacks are physically damaged, and I'm too lazy to desolder them and replace them, so they are out of the question.

Here's an illustration of how my receiver's speakers are connected:

And here's an illustration of how my computer is connected:

Solution 1:

OK, so unfortunately I ended up solving this problem like 6 hours after posting the bounty, so that was 50 rep wasted…

Anyways, it wasn't a software problem, or at least not on Windows's end.

Here's what I did:

1. Go to Sound --> Playback --> Realtek Digital Output --> Properties --> Supported Formats and select only Dolby Digital.

2. Select |> Test.
3. While Windows is sending the test signal, set the receiver to Auto Sound Digital mode, and the Listen Mode to Pro Logic.
4. After test completes, click Yes.

I can now listen to anything I want in 5.1 (ish - see below) surround.

The only issue is that the two rear speakers (the surrounds) are set to play the same signal, so it's really 4.1 surround (FrontR + FrontL + Center + Surround + Sub). This isn't too much of an issue, as it already sounds way awesomer.

I hope this helps someone else, and I'm willing to give the bounty away to somebody (post an answer to get put in the drawing - up votes to Question/Answer appreciated to help recover -50).

UPDATE:

It will play true 5.1 surround given a 5.1 source, like a DVD or other movie. But music seems to get 4.1, unless specifically written for 5.1.

And the bounty expired while I was out of town. So +50 really wasted.