Unlike standard software, Audacity does not automatically install FFmpeg. Many users download only the main Audacity setup file, unaware that FFmpeg is a separate add-on.
Some users have reported that swapping different versions of avformat-55.dll (say, from FFmpeg 2.2 vs 2.4) changes which formats work. It’s a weird, low-level compatibility game — the audio equivalent of tuning a carburetor. audacity avformat-55.dll