You connect what you have.
2.1 sound system = connect to front left right and center (All sound goes on 3 speakers)
5.1 sound system = connect fornt left right center rear right left (All sound goes on 5 speakers
7.1... you get the point... set sound output to sound system you have and you are done
I've got 2.0 on 7.1 card
it works perfect.