I remember I had some problems with some films. Make sure you have the correct AC3 codecs installed, but you probably have due to the audio working in VLC. Is the dialogue the main missing component. I found I had to change my speakers configuration in windows Sound control panel as I only have 4.1 (4.1 THX though!). So play about with the speakers and the dialogue should shoot up in volume, well, it did for me anyway.