If it helps, my speakers crack and pop as well, and every once in a while they'll (what seems like) blow themselves up along with one of those pops. Then they'll come back to life later on, whether that's minutes or days depends on the occasion.
For me, its my mini-fridge in my (dorm) room that turns on and causes the cracking. Check out if there's anything like that around your speakers. My Furnace used to do the same thing for me. When it would start up, the speakers would crack.
Take a look around and see if there's anything turning on when it happens. It could be a possibility.
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