well i know that a dvd has a kind of encryption and i think that the media player itself can't really play a dvd without having another dvd-player-software installed. maybe that's the reason why you can't hear anything but if you can see the movie this should not be the problem.
i would try to open the movie with a dvd-software like PowerDVD
PS: sound boxes on?

PPS: why do you have a german media player while talking english?