To touch on the auditory exclusion, studies show that, during a lethal force encounter, you will not "hear" the shots. Yes, the DBs will still cause damage, but you would not be "startled" by the firing gun. So, perhaps his time would be better spent in preparing to put rounds on target as quickly as accurately possible, and looking for good hearing aids. For more info on this, and other physiological responses to expect during a fight for your life scenario, may I suggest Lt. Col. Dave Grossman's books "On Killing" and "On Combat". Great stuff in both books!
"You can't miss fast enough to win!"
"Smooth is fast if you practice to be fast! Slow is just f__ing slow!" Words of wisdom from C!