For this I would install some USB Audio stick for the owner to test. I would never sign up that it could be fixed since that is out of our control as to motherboard audio and drivers.
This is such a common fix we use that we carry this in our PC medic bags.
If you feel it's in warranty with the new board, you can pursue that.
I made a post on another forum about a month ago where I had a problem with sound glitching. Since then, we've mostly figured out that it was a problem with the hard drive, which we hadn't expected, but it was very old and after testing it with a spare and having no problems, we replaced it with a brand new one and the original glitchy audio went away. (We got a new motherboard prior to this but it changed nothing as far as we could tell.)
But now we have another problem. Any external audio playback device (speakers, headphones, etc) connected to the desktop will periodically make a sort of popping sound like you would hear from a loose connection, after which the audio will cut down in volume before slowly going back up to the original volume. In addition, my friends have reported hearing strange sounds coming from my microphone that coincide with these problems. We've tried all sorts of things and only two things have seemed to work around the problem.
First off, it's only physical connections. Bluetooth speakers seem to work just fine, even after extended use. Any physical connection of any kind to any audio jack has shown this problem, but Bluetooth works fine.
Secondly, this problem goes away entirely when I'm playing games on my desktop and they are the active window. I've tried it with three titles so far, all in fullscreen - Subnautica, Dead Cells, and Stardew Valley - and all of them have shown no popping sounds while running. This applies across the board. Whenever the game is running as the active window, ALL applications are free of popping sounds, and my friends don't report hearing terrible noises from my mic. But as soon as I close whatever game is running or even tab over to a different window, the popping sounds AND the weird mic noises instantly come back.
I'm completely lost as to what I should do here. I don't think it's the new motherboard since we weren't experiencing popping sounds when it was installed before the new hard drive, but we've tried updating every driver we can and trying alternative drivers as well with no success whatsoever. My specs are posted below. Any help would be IMMENSELY appreciated.
Windows 10 Home 64-bit
Intel Core i7 7740X @ 4.30GHz 38 °C
Kaby Lake 14nm Technology
16.0GB Dual-Channel Unknown @ 1066MHz (14-14-14-32)
Micro-Star International Co. Ltd X299 GAMING PRO CARBON AC (MS-7A95) (U3E1) 41 °C
4095MB NVIDIA GeForce GTX 960 (EVGA) 32 °C
931GB Western Digital WDC WD10EZEX-08WN4A0 (SATA ) 31 °C
hp DVD-RAM GH82N
Realtek High Definition Audio