DVD"s aren"t AVI But VOB (Which is basicly just Mpeg-2 with AC3 audio) so converting VOB to AVI is a Bit time consumeing since you only need the audio....
What I use is a DVD Ripper called "Smartripper" and I go into the "Stream Processing" settings and just select the audio track you want and rip it to your Hard drive (It will probably be in Dolby AC3 format) and from there you can convert it to any format you want.....