What is your setup? Are you playing movie to a stereo system or 5.1 surround system, or ?
I've seen this when the movie has multiple sound tracks on different channels (like voice is on center channel, and music/sounds are on R-L channels) then if you play 5.1 movie to a stereo only source the center channel gets dropped out. Go to DSP settings (Tools>Options>Audio>DSP) and check the "output" settings. Is it set to "Source Channels" or "2 Channels" with "JRSS Mixing", or something else? Do you have "Bitstreaming" set to "Off"?