I would use YadeX:http://www.macetvideo.com/dl_center/dl_center.html
You just select what chapter or whatever and save as streams. If you click cancel on the video one, I think it will just save the audio. It will save a .ac3 file. You should convert this using a program like mAC3dec:http://sourceforge.net/projects/mac3dec/
into whatever format you like. You might be able to split the audio first using MPEG Streamclip. this way you aren't converting lots of audio.
If you need to split audio in a more advanaced way e.g viewing waveforms, you can try Audacity.