If you want surround sound on both your media center and your extenders here's what you need to do:
Install Divx Plus and buy it and register it.
Install AC3Splitter and go into the config and under main choose output format "AS IS (no change)". If you're using SPDIF or HDMI choose use SPDIF, then go to the SPDIF tab, choose USE SPDIF if you're usuging one of the above, and choose as is (no change) and pass through ac3, dts, and mpeg audio. Apply and you should be golden.
If you have any issues mess with the ac3splitter settings and you'll get it rolling.
The only other way to get it working reliably is through the shark codec pack and applying suggested defaults. However you get nasty toolbars and it's REALLY easy to break your entire setup so that WMC won't even play.
This way just works and works on extenders too. (xbox360 specifically tested)