you need to make sure whatever you're using to play it (be it a dvd player or software) isn't set to use 5.1 stereo, there is probably an option on the DVDs in a Setup menu that allows you to choose the audio setup.
inserting headphones like matt suggested WON'T do any good as you'd still be trying to play 5.1 sound through stereo speakers.
inserting headphones like matt suggested WON'T do any good as you'd still be trying to play 5.1 sound through stereo speakers.