here's a solution that should work for you, if you feel like buying extra stuff.
get a Sound Blaster Extigy (it's an external version of their Audigy, but can be used without a computer). then buy one of the advanced AV packs with fiber optic out. Connect the fiber optic output of the xbox into the fiber optic input of the Extigy (and an LED should light up to show you're receiving a DD 5.1 signal). You'll have to enable DD/DTS in the MS dashboard (audio settings) after you hook up that AV pack. The extigy has outputs for front speakers, rear speakers, and center/sub. That should work for your setup.