I managed to find a share-ware program called DVD Audio Extractor, here(link). It gives you a free 30-day trial, but after that costs you $30. The free trial period should be enough for this job. Just click 'Register Later' unless you want to buy it.
Using that in the single-file mode let me rip the audio from the VOB file to an MP3 file I could play. Some Internet pages say you can use the VLC Player to rip audio from a DVD or DVD file, but that didn't work for me - just converted the VOB into an MP4 video.
Still with VLC Player, Myguggi and Cdanteek have pointed out that early versions of VLC can give you some odd results with your MPG file, so it pays to use the latest VLC version, 3.0.6. Thanks for that, gentlemen.
Okay, now to this week's puzzle: Please compare these three screenshots -
The original MPG
My VOB file
Your VOB file
First thing to notice is the video has in both cases been processed by MyDVD from 6 000 kb/s to 9 000 kb/s, so that seems to have worked properly.
Next thing is that my audio has been converted to AC-3 at 192 kb/s, while yours doesn't seem to have been changed. Given that there aren't any options for the user to tweak, why did mine work while yours didn't? Further, your MPG plays sound so why doesn't your DVD with unchanged audio also play sound like the MPG? Could it be that your second Roxio installation isn't 100%, either?
CD and Myguggi, is there any chance either or both of you could try making an ISO from that MPG, and see what MyDVD does with the audio on your machines?