for f in *.flv; do  newname=`echo $f | tr ' ' '_' `;  mv "$f" $newname;  f=$newname;  mplayer $f -vo null -ao pcm:file=tmp.wav; lame -b 320 -q 0 tmp.wav $newname.mp3; rm tmp.wav; done

for f in *.flv; do  newname=`echo $f | tr ' ' '_' `;  mv "$f" $newname;  f=$newname;  mplayer $f -ao pcm:file=tmp.wav; lame -b 320 -q 0 tmp.wav $newname.mp3; rm tmp.wav; done

Script will extract all flv's or other videos specified to mp3.
You can change *.flv to prob. any other extension.
Tested on *.mp4 and *.flv

extract audio using mplayer

Bash, linux, convert

Bash