Page 1 of dad audio and picture not going together

hi all i recently rented undead from blockbuster and wen watchin in 5.1 the audio and picture wern`t going to gether but wen i changed it to 2.0 it was fine. why???

i have just bought dracula dead and loving it and that is in 2.0 eng and that started doing it aswell also my a few good men was doing it a while back aswell

this has happend on 2 diffrent dvd players

what causes it??