Surround Sound issue (ongoing)

So, been playing the game approaching a year now on PC and have had to live with it outputting a stereo sound instead of 5.1.

I have a Asus Xonar Phoebus installed with latest driver.
OS is Win7 64bit premium.
Soundcard is plugged to a receiver via optical cable .
I have tried adjusting settings in Windows and the Xonar program, as well as uninstalling and reinstall the game.

All have failed to yield a change.

Theoretically, The sound card itself SHOULD support real-time encoding of the game's LPCM signal to a Dolby Digital 5.1 signal that my receiver can then output.
This doesn't happen, however.

I know the card itself works, playing other games in surround, no problem .

Was hoping to get anyone's opinion and solutions to this , if possible.

Many thanks