They're compressed with the xbcompress tool from the Xbox SDK.
They need to be decompressed first. I tried using the tool but version 2.0.7645.0 was not the correct one. There is an earlier version out there but I couldn't locate it.
QuickBMS can apparently handle the decompression, but I have no idea how to write a script to instruct it to do so unfortunately.
Interesting. Hopefully someone will come along soon enough with a solution. I will check for a earlier version as well of the xbcompress tool from the Xbox SDK. Thanks anyways.