You said only the front two speakers are working in 5.1 surround sound mode but in stereo mode, all of the speakers are working.. Is that it ?
Did you try clicking the speakers individually if they create sound in this control panel, while in 5.1 ?
Are you sure you have wired the 3 wires properly ? Usually if all 5 speakers work in stereo, and not work in 5.1, that means they arent getting the appropriate sound signals directed at them due to faulty wire or improper connection..
The red port -> center and subwoofer
The blue port -> Back Left and right
The Green port -> Front Left and right
Try using the "Green Port" wire in the red port or blue port to test
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*dl.dropboxusercontent.com/u/72261408/settings.png
also try changing the settings here, one by one to see if they fix the issue