I just found that, if I have an AAC audio file (technically M4A), and if I added the ID3 tag 2 (aka ID3v2), then the audio file will failed to be converted by ffmpeg.
It can be either converted to mp3
- using DeadBeef audio player, or
- remove the ID3v2 tag then convert
So, how to add the metadata like ID3 tag? Use the Kid3 and add the Tag 3 (aka APE tag). This will not affect how ffmpeg to read the file.