I have an mp4 video that has no audio/video sync issue. I tried to extract audio to an mp3 file, following the suggestion in this thread: Extracting audio from MP4 video into MP3
ffmpeg -i input_file.mp4 -vn -abr 128k -acodec libmp3lame output_file.mp3
However, the mp3 end up to be 7:51 long (while the video is 3:26). When I played the mp3, the player will terminate at 3:26. Why is this the case?
By the way, I also tried to use Mac builtin encoder to encode the mp4 to m4a (Finder->Services->Encode selected video file->Audio only), and there is no problem. The output m4a is 3:26 long.