My Realtek drivers are up to date.

Also my brother has exactly same specs and same problem, so I don't think that problem is specific to my board

This is the first time I have heard anything like this about ASRock's boards. ASRock's Audio implementation (on this and many other boards) is isolated from the rest of the PCB and even has the left and right channels on separate PCB layers, add to that Nichicon audio caps and gold plated jacks and you have a very robust (for integrated audio) setup. 

It sounds like you are somehow picking up the polling of the mouse and some other form of electronic interference. Those speakers look pretty nice, I notice they also have headphone out so try this:

Connect headphones directly to the back of the PC and test for interference, if none then try hook up the speakers and connect the same headphones to the speaker headphone out.

Have you tried the front panel audio outputs?

I have the Z270 version of your board and have not experienced any issues with it. To the best of my knowledge the audio implementation is identical. 

So this is kind of partial fix. Cabling looks terrible but at least buzzing is gone.

It still bothers me that rear port is unusable.

Is there anything else I could try to do?

someone else seems to have same problem

https://www.hardwareluxx.de/community/f12/asrock-fatal1ty-z370-gaming-k6-spulenfiepen-1179353.html

Atleast what I understood from google translation to english.

Try moving around your USB devices (what ports they are connected to) as well as disconnecting them and see if maybe one of the cables isn't sufficiently shielded. I would start with connecting the mouse to the front panel ports and see if you still get the polling sound through the speakers. Dealing with audio can be tricky, any number of things can cause interference. For example I had to move my daughters PC away from one of the walls in her room because the AC unit was mounted on that wall outside and it would cause her audio to pop and crackle every time it turned on, off or changed speed. Now the PC is a few meters away and the problem is gone. 

Basically try moving things around your desk, especially things like power supplies and see if anything you have hooked up is creating interference and try to rule out the other devices connected to your system. The fact that the front panel audio is clean suggests that there is something external interfering with the signal from the rear, the connections for the front go through the same preamp (for headphones) and come from the same audio circuits. 

If you don't mind spending a little money on the issue you can try picking up a front panel audio header that screws into a free PCI slot on the back of the PC. I prefer doing this on my systems so all my audio outputs are at the back of the PC and cables can be kept tidier. If I need headphones I use an extension cable, much neater.

I appreciate that you tried to help. However in the end only way to remove the buzzing noise is to connect my speakers/headset to front audio jack.

If you are up for the challenge, you can try pulling the PSU out of the system and move it as far away from the board as your leads allow and see if you still get noise. Its also worth checking if your board is shorting onto something under it (like misplaced standoffs). If there is contact between the case and one of the audio circuits the case can act as an antenna, picking up and amplifying electrical noise.

You can also contact tech support directly from ASRock's website, be sure to set your region by clicking the round gridded icon at the top right of the page. Setting your region will give you the nearest tech support department to your country/location. If there is a known issue with your model board they will know about it and advise you.

I hope this helps

By the sound of it, it looks like it could be a case of Microsoft strikes again. Hopefully your problem remains solved. On the up side, if there was a hardware issue then it shouldn't be working correctly at all, for any period of time. 

I don't understand why fractal does not give any additional standoffs...

I hope this helps if others have same kind of problems

Is your OS install a fresh installation or did you move it over from a previous build? 

I get the feeling you are not having the same issue as Darqor. Your issue sounds more driver oriented. 

In that case I would recommend you contact Tech Support Directly with your issue. They will be able to advise you on the issue.

Go to the ASRock website, click the round cross hatched icon at the top right of the page and select the nearest region to your location then click support and go from there. By doing this you will reach the best suited support department to help you.

Please let us know how it goes.

I noticed that reducing/limiting the 'Power Target' from 100% to the minimum of 44% in EVGA Precision X (I have a GTX 1080Ti FTW3) the buzzing sound was drastically reduced. This told me it was definitely interference with the video card as it was drawing more power as I suspected. When I removed my video card I realized the card sits right on top of the SoundBlaster gold capacitors, doh! Put the card in the upper most slot and voila, no more buzzing in games or when there's a GPU power draw!

Seems I bought the wrong board... :(

Board is mounted on all 9 standoffs. Power Supply is far far away and one of the best you can buy (Seasonic Platinum Series). Tried the cleaning driver method... Nothing works.

Horrible buzzing and cracking while surfing in windows.

Only solution: Using again my old Asus Xonar DX Soundcard - SILENCE! But one of the reasons for the K6 was the sound because the Xonar DX doesnt get drivers anymore and I experienced some Win10 Bugs before... :( :(

From reading many hours on other manufacturer forums for similar z370 intel boards with ALC1220, it seems this intermittent 'popping' and 'crackling' sound interference is indeed widespread.
Of course the blame game begins, and it's never got anything to do with the mobo manufacturer - it's like they all think they are these completely innocent bystanders, yet they manufactured the boards and should have a responsibility to their customers to ensure these issues don't happen.

There's so many potential variable and I've lost days trying to resolve this.

Microsoft generic HD Audio drivers seem to be the most reliable, but still exhibit the same issues here and there. The Microsoft drivers at least allow the front audio panel (headphones for most users) to work without screeching distortion.
And one big difference - the Microsoft driver actually show two separate entries in Windows for headphones connected to the front audio panel and desktop speakers connected to the rear motherboard 'green' connection. Not sure why Realtek took this very basic feature away, so if you have both speakers and headphones connected they just show as one entry "speakers" and you have to physically remove the headphone jack from your front panel if you want to use your desktop speakers when you should be able to simply disable them when not in use from Windows sound settings.

Similarly, much older Realtek drivers seems to be quite close to the Microsoft drivers in terms of severity and frequency of these sound issues.

I honestly believe the best solution is to buy a cheaper PCIe sound card (Xonar DGX) and just be done with it and get back to gaming and the things you enjoy.

Is this a great solution? No, because we purchased these boards because of all the promises about great sound and EMI shielding and Japanese capacitors etc.
But AsRock literally refuses to acknowledge this is an issue, at least the guys in the Asus forums have admitted something is wrong.

But they can't fix it either, and it's tough to find a compatible board that doesn't have Realtek.
 

I am currently using Realtek v8210 that I found on softpedia or something like that. It has been mostly ok, but still the occasional pop and click in music playback. Don't notice it much in games likely because there's so many other loud noises in the games I play.

Good luck.