I know that this has had one mention in passing before, but...
On Hart Shafer's Audition blog, he mentions that somebody told him about an AAC/MP4 Audition codec plugin that appears to work fine. You can find it
here if you scroll down the page. Personally I have no use for this at present, but I know that some people are quite interested in being able to do this, so could we perhaps have some feedback from anybody trying it, or who already has?
I
thought I tried in AA v2 and it didn't work at all. I can't remember whether or not it crashed AA. I did use this codec in the past with v1.5, with some success (for decoding only). I also recall trying to use it to encode a file once. It apparently worked (no errors from AA), but the resulting file, taken elsewhere, was completely unplayable.
This is all vague and anecdotal, I know. But the fact that I've only had partial success with third party filters for AA, and the fact that these filters seems to break whenever there's a new version of AA is why I'd rather Adobe would support these not completely uncommon formats directly. Particularly when they still support so many obscure, obsolete formats.