You probably can't load the assembly ffmpegwith libfdk_aac support.
From ffmpeg LICENSE.md:
incompatible libraries
Ara Fraunhofer AAC library (libfdk_aac), FAAC, and aacplus are under licenses that are incompatible with GPLv2 and v3. We do not know for sure if their licenses are compatible with LGPL.
, --enable-nonfree . , , , , LGPL, . , .
() ffmpeg . ffmpeg Windows, Zeranoe FFmpeg FFmpeg Wiki: .