Be sure the Digital on your speakers configuration is set OFF
and speakersystem 4.1 not 5.1 or 6.1
I suppose you have only a green and black plug from your speakers
Are they connected like this ?
Green in 1 (front + subwoofer)
Black in 2 (rearà
http://www.bigred.co.uk/Preview/TMPqdwz1jx23c.htm
They are not the same as your Soundcard suggests