Question Asus Z720-K MB, Sound Blaster Audigy Rx: 'Line-in' and 'What U Hear' are now - and remain - "currently unavailable"...

[CoCo_Axial]

As per the title, I no longer can record using 'Line-in' and 'What U Hear': both are showing as "currently unavailable".
These options were working for years, up until a while ago.
OS is Windows 10, updated, no particular issues.

So:
Uninstalled the Audigy's drivers (using Revo Uninstaller, as Creative's uninstall file mysteriously missing ) and reinstalled latest: no change.
Bought and installed a new Audigy Rx: no change.
Reformatted C drive*, reinstalled Windows 10, reinstalled the sound card drivers before any other software or updates. No change (both 'Line-in' and 'What U Hear' showing as "currently unavailable").

Given that the sound card's apparently not the issue, and it's not an OS conflict with some other program, the only other cause I can think of is that I recently updated the mobo's firmware (from the BIOS using Asus's EZ Flash 3).
I've had a look around the BIOS and can't see any setting that may be responsible.
Any bright ideas?
For instance, is it possible (or wise) to revert the mobo to previous firmware?

Observations welcomed!
Thanks for reading.

* After making an image of the C drive using Acronis True Image, so I could simply revert to it. I also saved an image of the basic install, so i can go back to that too.

 

[CoCo_Axial]

Mystery deepens:

We have another PC using the same motherboard. The BIOS on that one hasn't been updated: it's 1804 x64 (7 June 2016).
I installed the new Audigy Rx on that. Same issue: 'Line-in' and 'What U Hear' showing as "currently unavailable".

The BIOS on my updated PC is 1207 x64 (22 June 2018).

So as far as I can see, this 'coincidental' glitch isn't down to:
a BIOS update
software conflict
a faulty soundcard
a faulty motherboard
a Windows update*

* When I reinstalled W10 I didn't allow it to update, so unless one was performed during installation...

What am I missing here?

 

[CoCo_Axial]

C'mon! Seventy views and no-one's got a clue?
This issue appears to defy rationality, but there's got to be some reason for it.
From where I'm sitting, the only remaining recourse appears to be exorcism.
Any alternative ideas, do please share!

 

[CoCo_Axial]

Posted this mystery on two other forums.
Numerous reads (120 here!), zero replies.
Which I take as indicating that what I'm describing is an apparent technical impossibility.
Except what I'm describing is the way it is.

BTW, 'Microphone' is also now showing as 'unavailable'. So now I can't record anything at all.