This was my thought as well. The sound is a red herring. A large enough EM blast would polarize anything in that room that could work like an antenna e.g. key

This was my thought as well. The sound is a red herring. A large enough EM blast would polarize anything in that room that could work like an antenna e.g. keys, paper clips, anything. Add a magnet near by or even another paper clip at a different polarization angle and you get vibration.