ive a couple solutions. well, one solution really, and one if-it-fits-you.
use xbmp (provided you have a modded xbox). it allows you to offset the audio in realtime, if the audio continues to get more and more out of synch you just keep adjusting. a really great feature.
the other - i watched a video the other day and needed an ac3 codec. i cant remember where i found the one i got, but in its properties i can set an audio offset. of course, this only applies to videos with an ac3 audio track.