I'm thinking that it has to be something electrical (random power surges?) or external (some unknown device trying to connect?)... but would anything like this make that sound?
To me this is clearly an Audio file/sound effect being actively played. power surges or electrical oddities don't typicall result in well-formed sound effects- but rather noise, scratchiness, etc. Though I could of course be wrong.
I couldn't find any sound effects that matched, though there isn't any sort of web search for audio clips which could be useful to figure this out. I tried a bunch of applications to see if any sound effects matched and looked up some website notification sounds- which also had come up empty.
here's some thinking to perhaps narrow it down:
Mute the System audio entirely from the Audio settings (Volume Mixer). Of course this means you'll have to deal with no sound but if you hear the odd sound, then you've completely eliminated any and all software causes from the system itself. If you still hear the sound, then it must be "down the line" and isn't related to the computer. If you don't hear it- or at least don't hear it frequently enough to be reasonably sure that it's gone- then it is coming from the System itself, so likely has a software cause.
If it is a software cause then you'd have to take the Volume Mixer approach and try to catch the audio session that is making the audio. I can't seem to find any tools to "log" audio session data (eg to track that say the blah.exe audio session was making sound at such and such time, and I doubt if I was to start such a project it would be usable before you went completely bonkers from the sound effect)
If it is hardware- that is, muting the system itself doesn't get rid of the sound, then it is between the system and the speakers. The DAC, the Speakers themselves, etc; in that case you could try swapping in a simple set of headphones or speakers temporarily to see if the sound effect goes away. If it does, it may be related to features like Bluetooth. on the speakers, for example- perhaps it plays it's own notification when it seems possible pairable devices, for example. Of course- as you mention, all that hardware changed, but I am guessing it largely changed from similar previous hardware. Best to go with as simple as possible and work your way up.
There is definitely something unique about your situation- I've never heard of people having this problem and I haven't experienced it myself- so Best to start somewhere.
Some additional thoughts
-Perhaps a list of installed software? Maybe something might be a clue there.
-as mentioned, there must be something unique to your setup, though no doubt you've figured that much out! As I understand, NUC devices usually connect via HDMI; what are you connecting to, for example? Has that display been changed? Are the speakers running through said display (eg HDMI->Display->DAC->Speakers)? Perhaps the display is emitting some silly notification noise. There is some common element between the systems that you've seen affected that perhaps you've overlooked as not having the capability to cause this, but in many cases hardware has been given a lot of weird capabilities that don't make much sense (A Display pushing notification sounds via the audio out mixed with whatever it got via HDMI wouldn't surprise me at all).
