In the interest of promoting fair use, I'm copying these instructions for converting a .aa file to mp3.

Why I Won't Be Adding Audible.com to My Xmas Card List:
HOW TO FIX THE PROBLEM 2004-04-27 16:28:58 ZenKai [Reply | View]

Okay, first things first. He's right. Dead right. And this is almost what made me quit when it came to Audible. Fortunately there is a solution:

Steps:
1. Uninstall Audible manager (no, I'm not kidding).
1.1 NOTE: Disconnect any devices prior to uninstalling.
1.2 NOTE: SAVE YOUR BOOKS or it will kill them too.
2. Download Goldwave 5.06 (this is shareware, though I strongly recommend registering; it's a L337 piece). Link at end of message.
3. Download and install LAME MP3. Goldwave keys off this open-source MP3 software to save mp3's. Link at end of message.
4. Install Goldwave.
5. Download and install Audible Manager 3.5. IT MUST BE v3.5! Link at end of message.
6. Open Goldwave, then open your *.aa file.
7. It will ask for your audible UID & PWD.
8. Allow the file to open.
9. If you are going to split the file, I recommend doing it now by openning the cue dialogue, choosing autocue, then using the Split File option to save to the proper format - no need to save to MP3 first, but you do need to set the default save options correctly)
Otherwise, Save as MP3. Figure on about a half hour per 8 hours of audio.
10. Burn, split, play to your hearts content.

Necessary search terms:
GoldWave v.5.06
LAME MP3
Audible Manager 3.5