can someone help me with a video question has to do with mp4 avi and divx?

[americansavage18]

hi guys i just downloaded a movie and i tried to convert it with a program last night (avi to ipod) and it didnt have sound. so i put it in divx and its says "this file contains a track in the dolby ac3 audio code 8192 format you may need to install directshow"

so my questions are how do i get the sound to play and can i convert the original file to mp4 and have the sound work so i can put it on my ipod?