When Captain Beatty comes to the house, Mildred had been watching her t.v. walls for quite some time. Montag had even asked her to turn the noise down, because he wasn't feeling well, and wanted some peace and quiet. Mildred, even though she said she would, didn't turn the volume down. But, Captain Beatty comes to the house--here is a man who is Montag's boss, making an unprecedented visit to their house. He walks in and has an air of authority, demanding that Mildred "Shut the 'relatives' up." It's the first thing out of his mouth, and is done directly, and with force. It's a bit intimidating, and Mildred was already nervous about him even being there. So,
"this time, Mildred ran. The yammering voices stopped yelling in the parlor."
Mildred was nervous; Montag had been acting pretty strangely lately, and that morning had refused to go to work. This upset Millie's world, and Beatty coming to the house probably made her assume that something was seriously wrong, or that he was going to fire Montag. So, her fear of the situation, in combination with Beatty's commanding demeanor, prompted her flight into the parlor to turn the t.v. walls off. I hope that those thoughts help a bit; good luck!