The person that called it in may have been inside and thought that they just wern't firing rounds at the time, hence no sound. (I.e. adjusting something) It could have been something else. It doesn't really mention much about the caller's whereabouts.
What we do know about the situation is that a man was saw holding what could have been VERY dangerous weapon, and was arrested.
"A man only loses everything when you cannot trust him." Helbino