Since DRM protection is used for Audible, the company probably uses different DRM keys for its customers, so the .AA files are different. Content providers can also use different ID3 tags in .AA files to identify customers. So to verify whether Audible uses watermarks or not, you need to remove DRM protection and make sure the files have identical ID3 tags. It is better to remove DRM protection with the same software. Then you can compare DRM-free files with the identical ID3 tags.