How do you extract the audio track from a video clip?

What platform are you on (Mac or PC)? What software are you using (or do you not have any)?

With any platform using QuickTime Pro open video file, go File export, select AIFF. It may take a while. If a clip of the audio is needed, open AIFF in QuickTime Pro drag arrows to portion desired to cut or keep. Open New and paste. To compress to MP3 or burn to CD open a free copy of iTunes. Open preferences advanced import, change to desired compression rate and file format, such as MP3. Select imported AIFF in iTunes library. Menu advance convert to MP3.

Done.

Note some video files may need to be converted to a more friendly file format.

I have both mac and pc. I don't have any media editing software other than those comes with mac.

If you don't want to buy QuickTime Pro, you can you iMovie:
Create a new project
Import your movie
Drag the clip to your timeline
Select the clip and click Advanced ==> Extract audio
Then save and exit.
Now you can browse to your home folder, then to your movies folder.
Instead of double clicking on your movie, ctrl-click and click select "show package contents"
From there, click on media, and you should see the extracted audio as an aiff file. See extolsmith's instructions for using iTunes to convert that to your filetype of choice.

mencoder (http://mplayerhq.hu/) will do it quite easily, from almost any media, but is command line =)