Had couple m4a files created by youtube-dl can not being played in itunes in MacOS. Used ffmpeg in CentOS to verter them to Mp3 withour any issues. Here is the command I used:
ffmpeg -i in.m4a -acodec mp3 -ac 2 -ab 320k out.mp3
For batch converting:
mkdir mp3s
for f in *.m4a; do ffmpeg -i "$f" -acodec mp3 -ac 2 -ab 320k mp3s/"${f%.m4a}.mp3"; done
