The audio streams present on a DVD can be there for many different purposes.
They could be foreign language versions, director's commentary, actors commentary, descriptive versions for people with visual impairment etc etc.
All of which can be in surround sound, mono, stereo or a number of other speaker variants in between so you can't tell what purpose the audio serves just by looking at whether it's surround sound or otherwise.
Anyway, to cut a long story short, view the DVD in a software player and note which audio stream is the one you want. You can select the audio of your choice via the DVD menus first if required.
PowerDVD for example lists the audio streams in order regardless of whether they're specifically named (as director's commentary etc) or not.