DVDs contain the audiostream in a format introduced by Dolby Laboratories. This encoding is called AC3 (or Dolby Digital) features 5 discretely encoded channels which (in comparison to Dolby Surround) have full bandwidth and are not matrix encoded into a two channel system.

As with the video-part of a DVD this audio-format takes up quite some space on the DVD, so to have as much bitrate as possible available for the video we can decode the audio and then re-encode it to MP3 or OGG Vorbis. Which save a lot of space and on normal home-stereos will sound okay. (if you have an AC3 receiver you may consider muxing AC3 into the AVI).

But if you're low on space you should re-encode to MP3. There are two good quality ways possible to decode the AC3-stream.

[GraphEdit using Intervideo Decoders]
High Quality

[AC3DEC Freeware Decoder]
Good Quality. Easier to use