QuickTime needs a special codec to read the video in an .avi, and even then it cannot read the way an .avi embeds the mp3 audio. The audio must be extracted and spliced in as a separate track.

DivX Doctor II (versiontracker.com, or 3ivx.com) does this automatically. It requires the installation of the 3ivX codec or decoder to function.

If a file chokes DivX Dr, then you can use DivX Tools (also at versiontracker.com), but you have to have QT Pro to reassemble. Open the file in QT, extract the video track, then open the mp3 which DivX Tool has created. Copy and scale-paste into the video or vice versa. If the movie is out of synch, you can fix that with QT Mutator. If the movie plays strangely in QT Player, use some other one. (GraphicConverter has a nice movie viewer, but a lot of people don't know that. Just drag to the GC icon -- also helpful when the pixel ratio is incorrect).