Compare a ddx file from vanilla Oblivion to a dds from the PC version to see if ddx texures have extra headers. XMA files are similar to WMA, just with additional header info. Hopefully you can figure it all out, and it turns out there are tools to do the job already.
I found the XMA converter you were talking about.http://www.ctpax-x.ru/index.php?goto=files&show=24
For anyone who needs it just click the text that says Download only from entire site
There are no utilities for the DDX files right? If so then I will try to compare two files tonight.
Update: Just tried the XMA converter and i couldn't get it to work... I'll remember to update this once something actually works.
Update 2: XMAEncode.exe can do the job but it's in the Xbox SDK. If you can get your "hands" on it, use that.