Spot on. Frame 285 of the z film shows a guy still applauding enthusiastically just before the head shot. Don't remember his name but he's a Normandy veteran and still hasn't realised something has gone wrong at that stage.
Charles Brehm. He was a ranger on D-Day. Which means, I believe, he either went up the cliffs Pointe du Hoc. Or over the open beaches of Omaha Beach, as was depicted in the movie ?Saving Private Ryan?. Either way, a hell of a day.
But nobody seen in the Zapruder film is seen to react to shots being fired until z312, the headshot, except for the occupants of the limousine and the standing Secret Service agents in the follow up car.
People were just ignoring, and likely forgetting, the bangs they heard. They dismissed them as firecrackers or vehicle backfires. And continued applauding and concentrating on JFK and Jackie. It was a big deal to them and they were not in a mindset to get distracted.