I've heard that theory too, but as I'm fully blind and heard them, that's not convincing to me. A more convincing theory I've heard is that the sound comes from the strong/sudden magnetic impulses creating interference in the nerves that carry sound/sense impulses to your brain. Your brain processes that interference as sound. It might also explain why you can't hear them if you look away. If you point your head downward, or go inside, or are making lots of noise as you move around and do things, the interference is either blocked by the back of your skull if you're looking down, or drowned out by all of the other nerve impulses your body generates as you do things. So you can only hear this "sound" if you stand still with your face pointing upwards, the position most people take when they're looking at the northern lights.