Did you use iTunes to convert video? If so, change another video converter for a trial. cause iTunes cant convert muxed video, many users complaint that video converted by iTunes has no sound. You can refer here for more info about it.
http://docs.info.apple.com/article.html?artnum =302588
I recommend 'Wondershare Video to iPod Converter' to you, my friends recommend it to me, very easy to use and works great for me. This software can transfer converted video to iPod directly without iTunes, you needn't add it to iTunes then sync to iPod again, it also has 24*7 hours online technical support, really help. You may download it for a trial.
http://www.dvd-ripper-copy.com/download/video- iPod-download.php?sid=2
Here is a step by step guide about this software
http://www.dvd-guides.com/component/option,com _smf/Itemid,93/topic,13252.new#new
If you are Mac user, you can use 'handbrake', it's free and fit for Mac OS, here is the download link
http://handbrake.m0k.org
Here is a step by step guide about it
http://www.top5soft.com/tutorial/convert-dvd-t o-ipod-with-freeware.html
I hope it helps.