The problem with anyone in an Apple store trying to hear the issue is that Apple stores are almost always filled with many decibels of background chatter, music, etc, making the crackling iPhone X issue impossible to notice. The other problem is that the electronic diagnostic tests that the Apple Stores run on the crackling iPhone X's all come up with satisfactory results. In any case, my local store traded my original cracklng iPhone X for a new one, and when I took it home and got it all set up, the same problem occurred. Actually, it was worse than my original X, so I took it back and got my original phone back. I then called Apple and reached someone who contacted Apple engineering. They, too, sent me a new phone...but it was worse than my original phone too.
My guess is that the issue actually happens to ALL iPhone X models. It's just that some people never use the speakerphone and/or just don't have good enough hearing to hear the crackling.
Anyway......I'm of course still hoping for a solution. And it will probably be hardware-related, which will be a HUGE thing for Apple to admit to and put into place.