VOBs are containers and may contain several sound streams (e.g. AC3 5.1, AC3 2-ch directors comments, other languages, etc). Try displaying the audio/video on seperate tracks and make sure its not a flat line (i.e. nothing there). Also right-click the sound track as see if there's an option (at the top) to "switch current audio track" to select another sound channel (option is only visable if one exists).