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a Recent Windows update did this to my Z270. A fresh reinstall of Windows 10 solved the problem. Likely something to do with the sound driver. you could try and mess around uninstalling and...

The gtx 680 doesn't run pci 3.0, that is a glitch in gpu-z reporting it as such.

It indeed will work, but its not ideal. This will activate the nf200 chip and increase latencies with the 2 cards. for a 2 card system, your best bet is slots 1 and 3
for 3 cards its 1, 2, 4...