There was a motorcade passing through the plaza and a large crowd cheering the President. The rifle shot was not loud enough in those conditions to attract attention.
Even so, the number of shots fired in the film were not hard to hear, as they identified both shots, and possibly a third, if one was only an echo. (The questionable one on the Police Department recorder picked up from the motorcyle cop's microphone, which was inadvertently left keyed.)
But I wasn't there, so can't speak with authority. Just guessing like everyone else. The number of theories we have on this is comensurately higher depending on who has the most information and how well they are able to integrate it.
I think it's humorous, personally. Everyone thinking they have better logic or have secret info nobody else has. Heh.