Although this beta does support .m4a, it's very basic when compared to .mp3 - I mean, "Listen" shows shows name, artist and album for .mp3 files - but only the basic filename for .m4a files
Unfortunately Sonata only supports ID3 tags, M4A files use a tagging system called QT tagging. Many months ago there was talk of a possibility of a tagging plugin model (to compliment the codec plugin model), but nothing has appeared I'm afraid, development resources in Roku seem to be focussed on the SoundBridge models.
I've actually written code to read the tags from a number of different file formats, including M4A (cover art as well), and this code has been incorporated into the 3rd party music player Sonica, so maybe you should give this a go (though it looks like you should use 2.0.34 as your firmware). Sonica doesn't need server software.
If you don't mind having a server running, then theres also SlimRoku which should handle AAC fine as well.