I've never heard of the expression tunnel hearing but I think I understand what is meant. I have heard a nearby gunshot unexpectedly and heard no more than a pop. Yet other times a car horn might startle me. I can't explain it but at least hardly anyone blows their horn anymore.
Tunnel Hearing is a term and effect I heard while looking at the 1986 FBI Miami shoot out. In an old documentary one of the agents discussed trying to call out and warn another agent. He produced that phrase.