Thanks to all of those who offered suggestions on how to rip the audio from DVDs. Here is what I ended up doing, all in Linux, should it be useful to anyone else:
First I used mplayer to identify the tracks on the disc:
myplayer -identify dvd://
Then I determined what the id number of the commentary track was from the list. On the Anchor Bay Tenebre DVD it was 131, for instance.
Then I ripped this track, outputting it as a .wav file:
mplayer -vid 0 -aid 131 -vo null -ao pcm:file=tenebre.wav dvd://
Then I converted the .wav file to mp3 using LAME:
lame -h tenebre.wav tenebre.mp3