Forgot to ask about this part, "fed up at last of hearing her, drew back a hand and struck her mother sharply about the face" and you also say that she can't hear what they are saying, so how can she hear a slap?
Author's response
...way to fail hard at reading comprehension. She didn't hear anything. Her father heard her mother, and slapped her. Nowhere in there does this equate to Toph hearing things.