I have had the same problem. I found that if restore the iPod then manage my iPod manually, my audible files work fine. If I use Automatic update my audible files stop working. I figured this out when my wifes ipod was playing audible files fine and mine had stopped working. She manually managed her iPod (she has way to many mp3s), and I was automatically managing my iPod.
Hope this helps

The Audible files on my 15GB iPod stop working from time to time as well. And I've never installed a Mac OS on it. I'm running 10.2.6 on my 800 FP iMac, managing my iPod manually, and still have to occasionally recopy Audible files to the iPod. I suppose we should mention it to Audible and/or Apple. But it's never been too big of an issue for me. Well, except for trying to remember where I left off listening. A bookmark feature would be nice. Or is there one of which I am woefully unaware?