this nice script will do it
call it youtube2mp3.sh and call it up like youtube2mp3 "url youtube" "title.mp3"
x=~/.youtube-dl-$RANDOM-$RANDOM.flv
youtube-dl --output=$x --format=18 "$1"
ffmpeg -i $x -acodec libmp3lame -ac 2 -ab 128k -vn -y "$2"
rm $x
prerequisites here; http://askubuntu.com/questions/218932/convert-youtube-to-mp3
