Well as mystifying as this all is, it's been going on with my Samsung S8+ since I got it, too. Just the S8+ has constant crackle going on, not my S7, S7 Edge-- those have occasional static, as does my iPhone 7 Plus. The only perfectly clear phone I own is the HTC 10, which makes everyone sound like we are all standing in the same room, always. So I'm guessing it's not the carriers.
Could it have anything at all to do with the plastic gasket type of thing that now connects between the glass and the aluminum frame?
The reason I wonder is that my. HTC 10 has all aluminum body like an IPhone 7 but has no water resistance whatsoever. No gasket. Nothing like that at all. All the other phones in question have some sort of sealing.
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That's been my experience, too, with the muffled sound. My HTC 10 and my iPhone SE do not have any kind of waterproofing and are both clear and static free. All of my phones with IP67-68 water resistance have some static. My Samsung S8+ crackles the worst. It's like talking over a bowl of Rice Krispies.