This IS a Microsoft problem. Windows 7, onboard Realtek 5.1 worked great, external SB 5.1 worked great. Window 10, neither my Asus Xonar 5.1 card, Sound Blaster Audigy FX 5.1, or Sound Blaster external 5.1 unit work on ANY Windows 10 machine. swapped out drive back to Win 7 drive in one system, 5.1 worked.
NONE of them would work with center, rear, or subwoofer test, even though they were a speaker option. Only front left and right. On the test it showed on screen sound was being sent to them, but no sound.
Now, 3 cards oout of three not working on anything? Yeah that is most definitely an MS problem, which means it's my problem now...