Dealey Plaza witnesses were around 50/50 or 40/60 on whether the three gunshots sounded like they came from the Depository or the knoll. The HSCA roped off Dealey Plaza in the 70's and fired gunshots from the sniper's nest and the knoll and had two observers stand around in several places and report their perception of the shots. They were about 90% correct in reporting the direction of the shots. This study have have even somehow been conducted firing real rounds, as they also have a portion where they test a "subsonic pistol".
This experiment should be replicated factoring in everything we know about the conditions at the time of the assassination (crowd noises, motorcycle backfire, firing real rounds into gel, more observers). But for now, what does the evidence show us? If we abandon the idea that a Carcano shooting from the sixth floor could cause half of 100+ witnesses to report shots coming from the knoll, then there are a few options:
1. There were shots from the knoll.
2. There was a diversion like a firecracker on the knoll.
3. Shots were fired from behind using a supersonic ammunition in conjunction with a noise-suppressor, which can create the auditory illusion that the gunfire originates from the opposite direction.
#3 sounds the most likely, but there are another couple of innocent explanations I've heard before:
The sound of the round colliding with Kennedy's skull created a noise almost as loud as an actual gunshot.
And also, Lillian Rogers' allegations of a seeing a woman's small child near the "black dog man" location throwing a glass coke bottle against the stockade fence.