I have the platinum version of DVDfab Last Ver 4 and most recent ver 5 and I am encoding movies with the (moblie) generic xvid+source audio. using both versions, The audio starts out fine (5.1 DD) for about and hour and thirty mins then it switches to 1 channel audio twords the end of the movie. I have done this to several movies with the same result.
This problem looks very strange, if you use audiocopy profile, DVDFab will change nothing for audio track.
What your mean is the your audio source change to 1 channel or you r player switch the audio to 1 channel ?
Did you try other DVD source ? not for some special Source ?
and Please try the new version and see what happened.