well what are you converting them from?
.avi? its probably not the program you are using to conver, sounds like its doing its job.

chances are since you dont see the video but see the sound, you're computer doesnt have the correct codec to play them. ill bet ya you can upload to your Ipod and it'll place.

just go to google and search for some mpeg4 codecs. or just go download DivX player, it should play it.