Muffled sound when playing dvd

Hey guys,
I have a 3500a
When playing regular dvd movies I only hear the backround sound of the movie clearly, but what the people are saying I can only hear a muffled sound. What can be the problem, also some dvds that I burn it doe not wanna recognize.
What can the problem with the muffled sound be?

dont know if this is the corect place in the frorum but any help is appreciated.
Thanks :slight_smile: