Suggest you look at this: http://www.halfgaar.net/surround-sound-in-linux
The 'testing' section at the bottom gives a thing that says 'rear right' and so on out of the appropriate speaker. Run that and see what happens.
The thing is, if my experience should be gone by, the computer will not use surround sound unless told to do so. So your surround sound may be working.
EDIT: There is also 'speaker-test' in the alsa-utils package.