Do you have the same sort of problem with other DVDs? It sounds like the DVD is damaged, perhaps scratched or something. Since it's always at the same point in playback that this happens, and no changes in the system settings have any effect, I would try a different Rob Roy DVD.
If you have the disk space, you might also try copying the DVD onto your HD. Then play it back from the HD, and see if the problem still occurs. I'm not 100% sure this is possible, but whenever I have trouble extracting audio files from CDs, copying the files to the HD first always works like a charm.