This script will find (including subdirectories) and convert all m4a files to mp3.

find -name "*.m4a" -exec ~/m4atomp3 {} \;

m4atomp3:

#!/bin/bash
m4afile=$@
avconv -i "$m4afile" -ab 320k "${m4afile/.m4a/.mp3}"

Remember to chmod +x m4atomp3.

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