#!/usr/bin/bash

for file in /media/nas_video/music/*.{mp4,m4a}
do
  if [ -f "$file" ]
  then
     export fdir=$(dirname "$file")
     export fext=${file##*.}
     export fname=$(basename "$file" ".$fext")
     echo "Processing '$fdir/$fname.$fext'..."
     ls -l "$fdir/$fname.$fext"
     ffmpeg -i "$fdir/$fname.$fext" "$fdir/$fname.mp3" && rm -f "$fdir/$fname.$fext" && ls -lh "$fdir/$fname.mp3"
  fi
done