I had the same issues when the phone first arrived with the stock chinese rom, heavy crackling on most sounds unless they were played from inside another app, even the ring tone.
Then I installed the xiaomi.eu rom about a month ago and the problem was almost gone, I still got crackling if I was accessing the UI and messing around but it was bearable.
Then the latest version completely solved the problem.
Mine has that same problem, but I am still running the (fake) Gearbest Rom, 8.0.5.0.0
That makes me even more eager to have the new stable Global ROM if that would solve what really is a software problem and not a high volume distortion as some have suggested.