Mormont’s raven saying Burn
The raven being a wildling skin changer theory aside, i think the emphasis on the raven screaming corn repeatedly while the Othor and Jafer debacle is going on is to maybe introduce a little doubt that the raven said burn. The words sound similar. Also technically birds can’t produce bilabials.
I personally believe the skin changer theory, partially because another way GRRM could’ve written Jon realizing fire was the answer would be for ghost to go near the fire or something else. Or even the raven flap near the fire then the drapes.