ffmpeg mp4 a mp3 | m4a a mp3

ffmpeg_mp4_a_mp3_m4a.sh

sudo apt-get install ffmpeg 

Trusty: 
sudo add-apt-repository ppa:kirillshkrogalev/ffmpeg-next -y && sudo apt-get update && sudo apt-get install ffmpeg -y


rm -Rf mp3/ ; rm *.mp3; for f in *.mp4; do ffmpeg -i "$f" "${f/%mp4/mp3}"; done && mkdir -p mp3/ && for f in *.mp3; do mv "$f" mp3/; done 

 rm *.mp4

En 320, sin borrar:
rm -Rf mp3/ ; rm *.mp3; for f in *.mp4; do ffmpeg  -i "$f" -ab 320k "${f/%mp4/mp3}"; done && mkdir -p mp3/ && for f in *.mp3; do mv "$f" mp3/; done 





for f in *.mp4; do ffmpeg -i "$f" "${f/%mp4/mp3}"; done

 find . -type f -name '*.mp4' -exec bash -c 'ffmpeg -i "$0" "${0/%mp4/mp3}"' '{}' \;

 ffmpeg -i "$f" "${f/%mp4/mp3}"

m4a
-------

avconv -i input.m4a ouptut.mp3

# for f in *.m4a; do avconv -i "$f" "$f".mp3; done

 find . -type f -name '*.m4a' -exec bash -c 'avconv -i "$0" "${0/%m4a/mp3}"' '{}' \;