Please do not cross-post in the other
forum if you are asking the same questions.
Why would you want to go through the trouble of transcoding audio? The space savings is not even worth the effort and you won't notice any visual quality difference. Besides, this is not the only thing you have to do. You must demux the video, 5.1 audio, subtitles, and mux them back in with the 2.0 audio. And then you have to replace them back into the original.
A bunch of guides for your reading pleasure:
http://www.videohelp.com/guides.
IMO,
DVDShrink would be the easy option for you.