All the in-ear earphones I've used with my Mi8 have this background "crackling" sound when I'm listening to audio. It's more obvious if the music is softer with fewer instruments in the background. For example, if the beginning of a song is instrumentless, then I can hear this krr-krr-krr sound, like very soft rattle-snake sound? At first I thought it had to do with the impedance of my earphones, but I've tried very high impedance earphones and still can hear the sound. My brother uses Mi8 and the problem is also present in his, so it's not something isolated. What to do?

My Mi A1 also has the same problem, but if I use wireless earphone, it does not has that "bizz" sound.

I am guessing it doesn't have a headphone jack, and to use normal earphones we need to plug in the white colored interface. Perhaps that's the problem?

If the earphones you are using did not come with the phone then it may be that they are incompatible. Earphones witha microphone hhave three black on the pin. These type are usually unsupported by many phones. You can try getting one with 2 black rings and find out if the sound is still there. Did it not occur to you that it may be your microphone that is recording that hiss sound.

Maybe you have the equaliser running with weird settings or it is not working properly. Check your phone to see if there is another equalizer that is running. If it find two then you have to disable one. Try as much as you can to eliminate all the music players that you have installed that did not come with your phone. Better yet, get a headphone instead. You may not notice the sound.