Make sure your sound output options match your speaker setup. On 5.1 output from a DVD, the voice is typically put out through the center channel only. If you output 5.1 audio to say 4.1 or two speaker setups, there is no center speaker and the voices will be lost, but most of the other sounds will be heard fine. This is can be fixed by changing various settings, depending where exactly the problem lies (DVD player settings or sound card settings).
Can you give more info? What program are you using to play your DVDs? What sound card do you have? What is your speaker setup?
