And your guess: AAX is a DRM-ed MP3 or AAC?
I believe that is a propritary format by Audible and yes, the Clip+ should be able to play them provided the player is set to MTP mode and has been authorized via Audible.
Audio: MP3, WMA, Secure WMA, Audible, OGG Vorbis, FLAC
My apologies ahead. But I think that’s the only supported ones for Sansa Clip+?
Yes. See that word “Audible”? AAX is an Audible format. Apology accepted.