Actually, the latest build adds support for tagging m4a files. ".m4a" is the commonly used filename extension for AAC files in MP4 containers. The use of the m4a filename extension was popularized by Apple. For instance, the files from iTunes shop are in the m4a/AAC format. Also the Apple lossless (often called ALAC) files are practically always in the MP4 containers (.m4a extension)
The format conversion to .m4a (AAC and ALAC) has always been possible by using the external encoder option. Though, until now it was not practically very useful because MC didn't have m4a tag writing or complete tag reading support (it could read only a few basic tags).
The new m4a input plugin, which has full m4a tagging support, changes this completely. AAC and ALAC encoding has become a viable and fully working option. An external encoder is still needed, but some high quality encoders are freely available and after such an encoder has been configured its use is as easy as the use of the other built-in encoders.
In the past I have posted instructions for using the free Nero AAC encoder. I will post a revised version of my instructions soon.
The encoder is available here:
http://www.nero.com/enu/technologies-aac-codec.htmlAnother option is to use Apple's free QuickTime codecs and the qaac frontend program. This combination can encode also ALAC files. I will create instructions also for this option. qaac is available here:
http://sites.google.com/site/qaacpage/