The title is significant in several ways. Melinda's inability to speak reflects her inability to tell anyone about her rape. It is such a horrific event that Melinda can't find the words to express what happened, much less how she feels about it. She somehow, as many rape victims do, blames herself in thinking she could have stopped it from happening.
The speech used by the characters in the book also show how people talk at each other rather than to each other. When the other students blame Melinda for calling the police, their speech is used to condemn her behavior and attitude. Even her parents, especially her mother, don't use speech as a tool for communication.
I think the title is also suggestive that there are other forms of communication besides talking that people miss. Melinda begins communicating through her art.
Melinda's ability to speak is related to her psychological well-being, suggesting that speech is so important to expressing our emotions. When we can't express them, we withdraw into ourselves. Right after the rape, Melinda begins her decline in her speech. The more she struggles by herself to come to terms with the rape, the less she's able to say. Melinda's urge to speak surfaces when she sees she has to help Rachel to prevent Andy from raping her as well. When she's able to tell about the rape, the more she's able to speak. Being able to share it with others brings back her speech and will lead to her healing.