mplayer -novideo -quiet -vc null -vo null -ao pcm videofile
[ OR ]
mplayer -novideo -quiet -vc null -vo null -ao pcm:file=audio.wav videofile
You can easily join the extracted audio with the audio description track using the Mencoder (MPlayer's Movie Encoder). Right now I haven't completed audio description for a feature length movie yet. My Gnome accessibility works takes a large chunk of my time. If you are interesting in working something like that, get in touch with me.