Usually I'd use vsound, I've been able do save the audio track of a video in realplayer, but for some reason it wasn't working for gxine.
Anthony Barker's blog had the concise steps needed to use mplayer to grab the sound.
mplayer -dumpfile outfile.dat -dumpstream some-video.wmv
mplayer -vo null -vc null -ao pcm -ao pcm:file=audio.wav outfile.dat
lame audio.wav audio.mp3
rm outfile.dat
