I recommend you read the WC Testimony of Lee Bowers. He directly addressed Sounds coming from the Triple Underpass vs Sounds coming from the TSBD. He disputes your premise as to a "difference in sounds of gunshots would be immediate apparent.............". Bowers not only worked for the railroad and manned that Tower for 15yrs, he also sand blasted the exterior of the TSBD. He was extremely familiar with the acoustics inside Dealey Plaza.
I have to disagree. Oswald could not have fired two shots bang bang in that short amount of time. I believe skinny holland's testimony, and the fake secret service agent Joe Marshall Smith encountered is enough proof something went down there.