The only thing I can think of is as follows :

Extract the audio as previously discussed, to an uncompressed wav file.

Open the avi file in Virtualdubmod, then go Streams->Stream List. Click Add, and add your wav file. Right-click on the mp3 audio track and click disable. Click OK.

Play back the video and look at how far out of sync it is, and whether or not is starts to early or too late.

Go back to the streams list. Right-click on the wav audio track and select Interleaving. In the section marked Audio Skew correction change the value in the Delay Audio Track by field. Click OK/OK and test. Repeat this until your audio is in sync. Save this out as a new avi (Direct Stream Processing mode for both video and audio), then use this new version in ConvertXtoDVD.