when i want to convert a avi using TMPGEnc i was getting no audio, i tried various audio rippers to extract the audio to a wav but it still did not work.
use CCE-SP.
http://www.cinemacraft.com
use standard settings and it will first create a wav file then start to convert the video,now just cancel it and you will be left with a wav file. this file will be larger then the original. using other rippers this was smaller, but did not work in TMPGEnc.