I am not at all sure I said that the OP was to blame. I did say that I have never had an issue with children speaking back to me, and that perhaps the OP might want to look at why the child speaks back to her. Just upthread she has since explained the child speaks like this to everyone. Again, this is not my own experience. My DH nephew was a freshie. A selfish and outspoken child who was not taught to share, to use manners, or to refrain from telling some adults which way the wind should blow. Fine. When he was with us explained what WE expected from him in terms of sharing, etc. and use of manners in a restaurant. His were shall we say, less than stellar. DH told him that it would be his choice to join us, however we would leave and take him home if he did notpolish those manner up immediately. It did not hurt that my nephew was with us and he told the boy "They mean it!" I never raised my voice, and still will not, however when we gave him a birthday gift and her threw it on the floor declaring "This is garbage!" I simply picked it up and placed it back in my bag, and said that Uncle and I were sorry he felt that way. The kid almost fell down. Mom and Dad looked shocked. I did not say a word nor did my DH and we took the gift back home. I never needed to ask for good manners again.
There are ways to teach children what your own expectations are, and while my tactics may not always work, they have a pretty good track record. I did have an epic fail, and that was a young undisciplined cousin of DH. When she was given the choice to come with us but with teh behaviors we expected, or stay home, she chose to stay home. She also was less than kind for her birthday gift, but she did nto improve. No more presents for her.