Realtek HD audio 5.1 optical input (from Xbox 360)

I'm trying to connect up my Xbox 360 digital audio to my 5.1 speakers via my PC (my LCD has dual input, DVI for the PC, D-SUB for the Xbox).

The motherboard (Abit AB9 QuadGT) has a Realtek ALC888 chipset and I have a 5.1 speaker system connected via 3 x 3.5mm jacks (FR/FL, RR/RL, C/LFE) and I get full 5.1 output from the PC.

I have connected the optical audio cable from the Xbox to the optical in on the motherboard's backplate.

With the Xbox in Digital Stereo mode I get 2 channel audio from the Xbox, through the PC, to the speakers.

With the Xbox in Dolby Digital 5.1 mode I get no sound at all.

I have the latest Realtek drivers installed in Win 7 32-bit.

Questions:

1. Is it possible to use the full 5.1 DD from the Xbox?

2. If so, am I missing some option(s) in the Realtek setup?

3. Do I need some other piece of software to do this? (AC3Filter or FFDShow perhaps)