Have you tried ripping it in another format then converting it to MP4 with iSquint? Such as MPEG or AVI? The MP4 tends to do that, I remember when iTunes first had the ability to convert video files to MP4, 90% of the time they would wind up with no sound. Unfortunately, iSquint has been disabled and is said not to work when you try and install it, but you could try downloading it from MacUpdate and see if it'll work for you. The source code has been released so there should be multiple open source alternatives before too long. Another way to convert it to MP4 would be with QuickTime Pro, but that is the slooooow route.
Eh, I just did to try it, and I was going to put a couple of my favorite movies on my computer, but if it has problems like this it was never that important to me in the first place. It was just annoying to wait like thirty min and then play it and have no sound.