Hearing aid (spoilers)
So the father, after trying for ages to build working hearing aids for his deaf daughter, finally builds one that works instead as a weapon against the monsters that drives them away, and this prevents a couple of fatal attacks during the movie. So by the end, they're putting two-and-two together and putting the hearing aid against an amplifier that stuns the monster in their tech room, but a fatal shotgun blast is still needed? And the silliest moment at the end is when they see other monsters come and the mother is cocking the shotgun like "LET'S ROCK!" and the movie ends before it turns into a monster bloodbath!
Good for the suspense, but it makes you wonder why no-one else in the world tried that sonic defence. As for the countries being crossed off that frequencies list, I can only assume all the humans were wiped out in them?