I have this problem as well. I'm not hopeful for an answer since I asked a similar question as far back as version 12 of Media Center and got no answer whatsoever. See link...
http://yabb.jriver.com/interact/index.php?topic=39940.0When I did this search another thread on aax files came up and one of the answers from Matt indicated that you're not putting much effort into audible anymore since there's little interest in it. Well, I am a person that has been interested in it from the beginning, but have never ever got it to work reliably on any machine with enough storage attached to make it interesting to me. I also would've wanted to be able to listen to them over the streaming media server to whatever client that I am using exactly like I do my MP3s. If there was lack of interest, I would suggest to you that it was because it didn't work reliably and technical support was just as lacking back then (i.e. when you were paying attention to it apparently) as it is now.
I, for one, would like to have Media Center manage all of my media files (including audible), but I've never been able to get it to work on a server class machine with enough storage. My current Media Center uses garden variety XP now as I learned my lesson on the Windows Server issue I had in 2005 on version 12 (i.e. I was told then that you didn't support Windows Server, while I suspect you didn't support audible reliably in truth), but it still won't work for downloads apparently in version 18 on XP (Perhaps XP is too old now... or is it audible you don't support... I'm confused?). So, although I'd love an answer here, I'm going to download the files into my Media Center file system using Audible Manager, let them Auto-Import and see if I have any functionality worth paying attention to once that is done.
Given the quality of the rest of the product, I've always found the poor performance of Media Center around audible troubling. I'm anxious for the MacIntosh version as well and will continue to buy it, but having all of my audible media managed outside of Media Center is not ideal to this long standing good customer.