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I've just been reading your post again, and realised that my previous answer, while technically correct, was not the right answer to your question. DVD is not the same as video files. DVD's have video vob files, and audio vob files. In order to play the right audio with the right video, you need a proper DVD player.
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<p>
openlp.org 1.2 will feature DVD playback, but until such time, you might want to use VLC or another DVD player.
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