# From "m4a" to "mp3".
ffmpeg -i input.m4a -c:v copy -c:a libmp3lame -q:a 4 output.mp3
# From "m4a" to "aac".
ffmpeg -i input.m4a -acodec copy output.aac
# Extract audio("mp3") from "mp4".
ffmpeg -i input-video.mp4 output-audio.mp3