To make MP3 Sushi work with AAC (.m4a .mp4) files, just edit the mime.types file, and change audio/mpeg mpga mp2 mp3 to audio/mpeg mpga mp2 mp3 mp4 m4a m4p. Then stop the jukebox server, and start it again.
To get to the mime.types file, control-click on the "MP3 Sushi Server" application, and choose "Show Package Contents" from the contextual menu. Then navigate to Contents -> Resources -> tools -> jukebox -> etc. You can use TextEdit, or any other pure text editor, to edit the file.
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