I suspect that this issue has little to do with the FFmpeg / Libav situation as neither modern version will work with Audacity. The situation is more that Audacity developers have not had the time to dedicate to keeping up with continuing changes from both camps. The emphasis of Audacity developers is doubtless on building and developing Audacity itself rather than trying to keep pace with the rapid changes in what is essentially a very sophisticated import and export plugin. Nice to have but not essential...
This guide attempts to give the FFmpeg functionality back to Audacity, at least for Trusty, until changes in Audacity will eventually render the guide redundant; could be a while though .
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