jeffro, can you get the software open now?
If the headphone slider doesn't adjust volume, then it's possible the software crashed and restarted. A reset will usually fix that. If it doesn't, then an uninstall, including removal of the folder I mentioned above, then reinstall should work. Another alternative is to do a hard shutdown to reset the amp (turn system off, then flip PSU off), then boot again to see if it's fixed.
On my X299 DARK, I have the card in the X4 slot. That's mostly due to other PCIe devices in slots to allow SLI to function at X16, but that's where I have it, just FYI.