Windows 10 surround sound test that's built into control panel works fine, as does AC3 encoded 5.1 surround test file: 5.1 surround AC3 test file
My setup is an Using MSI P67A-GD65 B3 with full analog cables to a receiver that goes to the speakers.
I have tried this YouTube test video: 5.1 test video on YouTube
It comes out in stereo.
It's possible this is impossible to fix? Netflix doesn't try to support it through a browser:
5.1 surround sound is not currently supported while streaming on a computer using Microsoft Silverlight or HTML5. --Netflix