Disappearing Audio



I have been a long time user of DVDFab Platinum and I have the latest release installed on my computer. I am proud to say that I have about 99.9% success rate in using this program. What I do is backup main movies only always at DVD5 and burn them onto a single layer disc. Occasionally I encode the main movie to AVI. XVID.AUDIOCOPY at 1500 KBPS with allmost 100% success rate.
But check this out. I encoded a “Untraceable” to AVI.XVID.AUDIOCOPY at 2000 KBPS. I thought everything went well but when I watched it, the audio disappeared about 3/4 down. I checked the main movie (the source) and the audio was fine all the way through. I re-encoded it again but this time I did it at 1500 kbps. It was perfect as usual.
Is there a logical explanation as to why the audio disappears at 2000 kbps?

Thanks to all.