I guess the first thing I have to point out is my total lack of use of XVID. That is a download format I find mostly unnecessary. It's a fad format that's probably not going to last (much like DVIX and REAL and some others). Fringe uses, not mainstream.
My initial thought is to try and separate the video and audio streams, the convert the audio in BESWEET to WAV PCM audio. Whether there is actually a tool out there or not, I'm not sure. Can you demultiplex MPEG-4 (XVID)? Probably, but finding the tool may take a little research.
This, of course, assumes it is VBR audio causing the issue.
Another simple trick may be using
VIRTUALDUB to extract the audio stream to a new file.
OPEN video file, then change the audio option to FULL PROCESS, then extract it as a new WAV file.
Please let me know how this works.
As I've mentioned to a few others recently, sorry for the delayed response. Was unavailable for a short time. But I'm back now.