So only the later shots were heard? For some reason.
The digitally enhanced Robert Hughes clip appears to show a person wearing a white ("tee"?) shirt standing and moving around in the Sniper's Nest window as the limo is turning onto Elm Street.
If Oswald was standing/crouching during his first, sharply-downward-angled shot, the muzzle of his short-rifle was probably just inside the building, making that shot sound somewhat muffled compared to his next two shots when the muzzle was outside the building.