Nonsense. JBC recognized the first shot as the sound of a high powered rifle. It was plenty loud.
Wow. 35 years research and this is the best you can do.
In all those years of intense study you must have noticed that different witnesses were located in different positions.
JBC had just passed the front of the TSBD building and was clearly in LoS of the shooter so, of course he would have heard the shot clearly. There are other witnesses positioned in front of the building who describe the shots as being very loud.
However, and here you might have to consult your copious notes made over the years, Zapruder and Sitzman were located on the west side of the building, not directly in front.
This affected how they heard the shots.
This is from Sitzman's Oral History interview given in '93:
"The last shot that we heard was right in front of us and it was like the same sound—
far off and to the left...
...the sound we heard… the third sound still
sounded a distance because if it had been as close as everybody’s trying to tell us...we would have jumped sky-high.
[referring to a young couple sat a few feet away] ...they were sitting in a park bench and they dropped their pop bottle on that cement there and cracked it. That’s what kind of woke us up, and that’s when we got down off of the concrete. But that sound was, like, five… eight feet from us. That, yeah, we did hear, but that’s the only other sound other than that
far away sound that we heard."
Here we have a witness, stood next to Zapruder, describing the sound of the shots as "far off".
Because you are a Tinfoil Nutter any witness that undermines or disproves any fraction of your spoon-fed beliefs about this case, is dismissed or are talking nonsense.
I've no doubt you believe you treat all the evidence fairly but this is not the case. Here is a witness who completely undermines your naive approach to the issue of the shots but, because you are so utterly deluded, you believe you know better

Funnily enough, in an interview with Josiah Thompson Sitzman disagrees with the notion of jiggle analysis, presumably because the sound of the shots was so distant it couldn't have caused a startled reaction.
Zapruder was an old man with vertigo stood on a narrow plinth taking a home movie. To expect a completely smooth tracking shot is ridiculous. Jiggle Analysis is over as a method of analysing the Z-film. Get over it.