Why aren't Atticus and Alexandra able to hear the kids in To Kill a Mockingbird?

This is somewhat hard to answer -- it would be easier for
us if you could give us some idea as to where in the book you are asking about. 
However, I will assume that you are asking about the events that happen late in the book
-- in the part where Mr. Ewell tries to kill Jem and
Scout.

In Chapter 29, Heck Tate is asking Atticus and the
kids questions about what happened when Ewell tried to kill the kids.  Scout says
something about Jem yelling really loudly when he thought that Cecil Jacobs was messing
around with them.  At that point, Tate asks Atticus if he heard the yelling.  He said
that he did not hear it and neither did Aunt Alexandra.  The reason that they did not
hear the yelling is because they were both listening to the radio.  Atticus was
listening to his radio in the living room and Alexandra was listening in her
room.

So the reason that they couldn't hear the kids is
because they were listening to the radio and the radios drowned out the sounds of the
kids yelling.