Question about DVD music

I bought some DVD music (‘The Planets’ - Holst). It arrived today (about 10 minutes ago, in fact).

I put it in my DVD player, and played it. I heard the music - this is OK. problem is - it seems to be coming from 2 speakers.

The main reason I bought a music DVD was to take advantage of the 5.1 music speakers I have. I played around with the settings a little and set the audio on my DVD player to 5.1 instead of ‘Enable sp/diff output’ (which is what it was on). this caused the music to come out of all 5 speakers. Trouble is - the sound card software causes all music to come out of 5 speakers in the event that it is only stereo.

Question - by selecting 5.1 in the DVD audio options am I utilizing the 5.1 sound of the DVD? or am I just telling the software to play the same at the back as is being played at the front?
I am very tired so I am sure none of that makes much sense. please try to extract some sense from it and figure out what I mean. Thanlks…