ffmpeg converts double length on MacOS

ffmpeg creates double length of mp3 from mp4 on MacOS ======================================================= Symptom -------- When you try to extract a sound file from a video file on MacOS, ```bash $ ffmpeg -i video.mp4 sound.mp3 ``` you'll get a sound file but it has double the length of the video, e.g. 10' in a video to 20' in a sound. Solution -------- You'd better put an option (`-write_xing 0`) to evade automatic treatment on a command line ```bash $ ffmpeg -i video.mp4 -write_xing 0 sound.mp3 ```