I think Amiga is on the right track here.

If you are attempting to play it on your computer, you will need to install a filter to handle Dolby Digital 5.1 sound. I usually recommend AC3Filter.

Another option, as Amiga said, is to download and use VLC player which contains all the necessary codecs internally.

If you are not getting sound from your standalone DVD player, have a look at the setup and see if it supports DD5.1 and also try a "Downmix" setting which should be somewhere in the setup.

This problem seems mor prevalent on Windows Vista rigs.