Asus Z720-K, SB Audigy Rx: 'Line-in', 'Microphone' and 'What U Hear' "unavailable"

As per the title, I no longer can record using 'Line-in', 'Microphone' and 'What U Hear': all are showing in 'Sound' (see pic) as "currently unavailable" (OR, as has recently happened, the "What U Hear" shows as "Ready" but in practice doesn't function when using recording software...)
These options were working for years, up until a while ago.
OS is Windows 10, updated, no particular issues.

So:
I 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: reinstalled drivers from CD: no change.

So:
Reformatted C drive*, reinstalled Windows 10, reinstalled the sound card drivers before any other software or updates (*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 new basic install, so i can go back to that too).
No change: both 'Line-in' and 'What U Hear' still showing as "currently unavailable".

So:
Given that the sound card's apparently not the issue, and it's not an OS conflict with some other program, I had a look around the BIOS but couldn't see any setting that may be responsible.
The only other cause I could think of is that I recently updated the mobo's firmware (from the BIOS using Asus's EZ Flash, to 1207 x64, dated 22 June 2018).

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

So:
As far as I can see, this 'coincidental' glitch isn't down to:
dodgy soundcard drivers
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, and I don't think one was performed during installation)

This issue appears to defy logic, 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!

 

Posted this mystery on two other forums.
Numerous reads, 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.