Asrock Fatal1ty Z370 Gaming K6 buzzing audio

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. 

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.

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