There are easier ways of doing it, but I've done it once with an Oasis DVD. I ripped the chapter with Smartripper, and I started encoding it with DVD2SVCD to get the .wav file (making sure the enabled the setting not to delete any files, because it immediately starts to overwrite the .wav to .mp2 otherwise). Then I used some command-line prog (I forgot the name, but you can get it from doom9.org) to convert the audio from 24-bit/96kHz to 16-bit/44.1kHz. Then encoded to MP3 with EAC/LAME, and MP3Gain to raise the gain to a listenable level.
EDIT: I don't know about Dolby Digital tracks, I did it with PCM. But I assume the audio prog in DVD2SVCD converts it for you.