Exporting to MP3 audio from Audacity works fine, but exporting to "M4A (AAC)" results in nothing. No file is created or whatsoever.
I can see that it depends on "ffmpeg" library to work, but the application WinFF (which also makes use of this library) is working fine to me. Do I need to install anything else?
This seems to be a known issue between ffmpeg and Audacity 1.3.x
Read more about it here.
My guess is that the Ubuntu packagers decided not to compile Audacity with support for the ffmpeg libraries because of these issues. I don't know what versions are in the official repo, as I'm using the Getdeb.net ones. I can confirm that what you said does not work for me either, but it applies to all ffmpeg exports.