The easiest would be a program called 'total recorder' though it wouldn't be the best quality. Set it to record, then play the part of the dvd you want, then stop it and edit the file in 'sound recorder' (start - a;l programs - accessories - entertainment)
The high quality way would be dvddecrypter -> dgindex -> besweet -> edit file
I recently did this for a friend. If I can remember correctly I used pgcdemux to grab the ac3 track from a dvd and then used besweet to convert it to mp3.
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