Convert mkv to mp3
$ find . -type f -name "*.mkv" -exec bash -c 'FILE="$1"; ffmpeg -i "${FILE}" -vn -c:a libmp3lame -y "${FILE%.mkv}.mp3";' _ '{}' \;
Convert webm to mp3
$ find . -type f -iname "*.webm" -exec bash -c 'FILE="$1"; ffmpeg -i "${FILE}" -vn -ab 128k -ar 44100 -y "${FILE%.webm}.mp3";' _ '{}' \;
Command to use it
curl -s -L https://raw.githubusercontent.com/julkwel/mp3-converter-linux/master/converter.sh | bash