Trusty Tahr 14.04 ships with EasyTag 2.1.10 and this release incorporates an important change dealing with m4a/aac tagging. Taken from the Ubuntu Changelog:
- Switch to TagLib for MP4 tag editing and drop libmp4v2 support
This change was introduced for Easytag 2.1.8-1 and gets around the licensing issues which made it difficult for Debian / Ubuntu to have a version of Easytag in the repository that could edit aac files.
So from Trusty Tahr onward there is no need for installing extensions, recompiling, adding PPAs, searching for other applications; to edit the metadata in m4a / aac files you simply need:
sudo apt-get install easytag
Happy tagging :)