sounds like you do not have the correct audio codecs installed on you comp or they are not present on your system. during the converting to avi the audio usually gets compressed into mp3 format, if your program you use to do the conversion does not have the codecs installed it will not be able to process the audio stream. here is an app that has all the popular audio and video codecs, its almost a necessity if your doing conversions.
AceMegaCodec D/L page
during install, you may want to select (pro) install and just scroll down to the bottom of the list and look for (Morgan Multimedia) and then unselect those 3 codecs, youll never need them and they're the only codec that adds anything to your comp and startup folder, also they seem to place a tray icon for their functions which puts my panties in a knot.
also if you are using one of those 'all-in-one' dvd ripper apps, you may not get as good a result or files sizes you may get from using 2 different steps to do the conversion.
step 1: would be to convert the dvd format (udf) into mpg2 format, which is smaller than udf but still quite large.
step 2: convert the mpg2 file to avi or any given format using (VirtualDub) which is a powerful opensource app, and better than alot of crap sold on the market. its free too at divx-digest.com.
if you need any help, let me know, im interested in seening the video too. i can host the file also as long as its not over 700mb.