I've had this issue too a couple of times on my Win2K computer (audio OK in the preview window, but no audio in the extracted backup files.) Both times the source DVD was a "musical."
As a test, I installed (temporarily) FAB5 on a friends WinXP computer, made a new backup, and the audio worked fine in the backup hard drive files and a DVD-R burned from those files.
I took the "no audio" hard drive files from the Win2K computer, and they played fine on the WinXP box.
I burned a DVD on the Win2K computer from the "no audio" hard drive files, and it played fine on the WinXP computer and a standard Sony DVD player, but not on the Win2K computer.
I don't know what the variable is that's the culprit -- if I learn anything more, I'll post again in this thread.