I am pleased to announce that a plug-in for encoding and decoding of M4A, M4B and AAC formats is available for beta testing here
Due to licensing reasons it's only a time limited (30 days) trial version, it will be possible to buy the full version soon for a small fee soon. There isn't any other limitation.
Don't forget that it's a beta version, we are interested in your feedback!
P.S.: This thread is intentionally locked, for any feedback please start new threads.