[xjsb125]

Maybe use something like DVD Decrypter to demux the vob into the elementary streams (.m2v for video; .ac3, .wav, mp2 for the audio). I can't remember if Premiere CS4 supports .m2v, though. If the audio is in .ac3 format, you may want to convert it to .wav to help sync the new source. From there you can put the new audio stream and the old video stream into Encore and create a brand new DVD if you like. You'll have to seek out other help to mux the files and drop it back into the current disc, I've never tinkered with it like that, but hopefully this will help you get off to a good start.