"The licenses of libfaac and libfdk_aac are not compatible with the GPL,
so the GPL does not permit distribution of binaries containing code
licensed under these licenses when GPL-licensed code is also included.
Therefore these encoders have been designated as "non-free", and you
cannot download a pre-built ffmpeg that supports them. This can be
resolved by compiling ffmpeg yourself."
See https://trac.ffmpeg.org/wiki/Encode/AAC.
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