Veleno, there are two lines of code, not one.
Btw, add -vo null to disable video play during audio extraction:
Explain:
mplayer - advanced media player
-ao (audio output) - select audio output
pcm - audio output will be redirected to WAV PCM file (defaults to audiodump.wav)
:file=audio.wav - override defaults and name file to audio.wav
-vo (video output) -select video output

lame - (LAME Ain't Mp3 Encoder) - mp3 encoder (not sure if installed by default, may be you'll need to install it via apt-get install lame)


And don't hesistate to use --help option of above commands and google.