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The only way you'll be able to do that is by ripping the soundtrack from the DVD, since DVD is video and MP3 is music.

If you mean converting a DVD into MP4 - you need a DVD ripper to get the files off the DVD, or if you already have the files ripped use something like VisualMonkey.

Whatever you do, don't rip ™ material you don't have any rights on.
 
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