What was Mildred probably saying here in Fahrenheit 451:
When she comes out of the parlor, she stands looking at Beatty and Montag and then her mouth moved and she was saying something but the sound covered it.
I think that what is going on here is that Mildred is asking Montag about the book that he has hidden. It's pretty dumb of her to ask in front of Beatty, but they get away with it. Here's how I know this.
A bit before the part you cite, she felt the book behind the pillow and she starts to ask what it is. But then Montag and Beatty are in the parlor and they are talking for a while. When she comes in and her mouth moves, Montag thinks it is lucky that Beatty can't see Mildred lest he be able to lipread what she is saying the way Montag can.
So Mildred is saying something that would be bad if Beatty knew about it. What could it be? It has to be about the book.