First of all, get Virtualdub, available here:
http://www.virtualdub.org/Now, open up virtualdub, click "
File" "
Open Video File" and select your file.
If the file is in AVI format, you can select
DIRECT STREAM COPY under both the Video and Audio tabs, if not, you'll need to set up the compression for video.

Under the audio tab, select "
Interleaving" and set your "
Audio skew correction" to whatever you think is needed (in milliseconds).

You can go to "
File" "
Preview" or press "F5" to preview the movie to check if the audio is right.
Alternatively, you can press up or down on the xbox's joypad in XBMP to fix the audio as well