This is truly a surreal scene that unfolds before Scout in front of the jail. Scout does not recognize the seriousness of the moment until many years later, since she does not realize why the man are there in the first place. The two carloads of men are actually a lynch mob who plan to take Tom Robinson from the jail to be hanged. They have dispatched Sheriff Tate on a "snipe hunt," leaving Atticus alone to stand guard over Tom. The men are also willing to hurt Atticus if he tries to stop them. Yet, their respect and admiration for Atticus becomes evident in the "sickeningly comic" but potentially deadly situation: When they demand to know if Tom is inside, Atticus tells them that he is, but that he is sleeping, and Atticus commands them to be quiet.
"Don't wake him up." (Chapter 15)
And the men, who are there to kill Tom--and possibly kill Atticus, too--immediately do as Atticus tells them.
In obedience to my father, there followed what I later realized was a sickeningly comic aspect of an unfunny situation: the men talked in near-whispers. (Chapter 15)
So, the resulting conversation is spoken in whispers so as not to awaken the prisoner who the lynch mob has come to murder.