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The Asus Xonar U7 sound card will output 5.1, but the Logitech Z906 speakers are stereo only. For 5.1 you need three more speakers, one for the center front, one for the right rear and one for the left rear. Also all of the speakers should be powered as the sound card does not have a large enough amplifier to drive the speakers directly since it has basically headphone amplifiers only.
What would be best for you to get 5.1 sound would be to get a complete 5.1 system made to work with the Asus Xonar U7 sound card. Like maybe this one: https://www.amazon.com/Logitech-Surr...puter+speakers