Large LPCM audio track!

I’m trying to backup some opera dvds I have. Many of these have a large (> 1.5gb) LPCM audio track, and so DVD Shrink or Recode can’t make it because they would have to reduce the video quality too much (in fact they won’t go below 50% quality) making the backup impossible.
I thought I could convert the audio to AC3 or even MP2… but how is this done? What software should I use? Will I need to re-author the dvd?
Thanks for any help.

try following this guide.

This looks exactly like what I needed!
Thanks AZ!