One tiny addition to Mr. Moore's URLs. The one he gave for convert2mp3 lands you on the German language page. Click here for their English language page.
If these services work for videos of the length you need to extract the audio from they are probably the easiest way to do it. The link I posted previously is to a How-To-Geek article that goes step-by-step through the process for VLC. But, I don't know how accessible it will be because it uses a lot of screen shots and I didn't take a look at whether there is alternate text or if the accompanying text is detailed enough. The author does give each and every dialog you need to go through, and it's quite a few of them.
