It can do both - decode and encode AMR files. Installation is simple - just copy the mmamr.flt file into the Adobe Audition directory and you should be good to go. The source code is a part of the amr_pack (http://dev.monogram.sk/websvn/listing.php?repname=amr_pack&path=%2Ftrunk%2F#_trunk_) repository.
Download : audition_amr.zip (264 KB) (http://blog.monogram.sk/janos/download/plugins/audition_amr.zip)
Enjoy,
Igor
People... please test it & report bugs.
Thanks.