The multiple audio streams you refer to are usually, but not always: the Dolby Digital 5.1 surround sound-track; DTS (Dolby Theatre System) sound-track; Director's Commentary and Alternative Director's Commentary (usually 2 channel audio) and language tracks (again in 2 channel audio) The language used for the main Dolby sound-track depends on the region encoding for the disc.
There are stream numbers associated with these tracks. You'll tend to find that:
- 0x80 is the main sound-track for the region of your disc
- 0x89 is used for DTS
- 0x82,83,84 and so on are usually director's commentaries or other language tracks
These tend to be standard across discs but can be authored in an arbitrary way meaning some discs don't adhere to these rules.
You should really check out some of the FAQs and guides not only for audio, but also general DVD structure. You'll find this of invaluable help to answer many of your future questions about DVD backup.