うまく行かない人はffmpegを最新にしてみるとか
Builds - Zeranoe FFmpeg
https://ffmpeg.zeranoe.com/builds/
直リン
https://ffmpeg.zeranoe.com/builds/win32/static/ffmpeg-4.0-win32-static.zip
https://ffmpeg.zeranoe.com/builds/win64/static/ffmpeg-4.0-win64-static.zip
XPの人はここ
https://sourceforge.net/projects/ffmpegwindowsbi/files/2016-08-12-v3.1.2/