Well, it cant be the drivers, since everyone has the same soundcards and other hardware (with the exception of the DVD Player) so maybe yours is faulty. Did you change any settings? Maybe you screwed a file up.
When your booting in does it have the sound file at the start?(im assuming Gentoo home, if its pro, then im not sure if one exists) and when you log off?
I think your best bet would be to re-ftp the files to your Xbox. (hopefully you backed them up, i didnt my first time and had to redownload it