Convert all found m4a to mp3

When reading a file line by line, if a command inside the loop also reads stdin, it can exhaust the input file.

Continue reading here: Bash FAQ 89

So the code should look like this:

find . -name '*.m4a' -print0 | while read -d '' -r file; do
  ffmpeg -i "$file" -n -acodec libmp3lame -ab 128k "${file%.m4a}.mp3" < /dev/null
done

Why not just use the -exec argument of find? So find -iname '*.m4a' -exec ffmpeg -i {} -n -acodec libmp3lame -ab 128k {}.mp3 \; and run a rename command afterward?