Theme statements should describe an argument, or the author's stance on a certain topic presented in the novel.

A theme statement regarding the topic of communication in Speak might be: "The theme of nonverbal communication throughout the Laurie Halse Anderson's novel Speak is a metaphor for the main character's secret, which she is never able to disclose aloud in the novel."

Another theme statement might discuss the theme of depression in Speak. For instance: "Author Laurie Halse Anderson explores several ways that teenagers deal with depression in the novel Speak, including self-expression through art, self-mutilation or cutting, and various types of friendships."

In any theme statement, make sure you explain clearly what you believe the author's point of view to be, as well as showing what the "topic" is, and phrasing the statement as an argument that can be proven.

One thematic statement from Anderson's Speak is that isolation can allow people to better understand themselves.

Melinda goes through a horrific event the summer before her freshman year. She is unable to fully process it and its impact until she isolates herself from the rest of the world.   She is not able to do this when she is in the company of others.  Cutting herself from others  enables Melinda to understand the implications of what happened to her.  Melinda's isolation is what allows her to figure out what is important and how to "speak." 

Isolation is what enables Melinda to grow.  The outside world does not "get" her.  As Melinda becomes increasingly isolated from others, she endures their scorn.  Yet, Anderson is suggesting that being apart from the clamor of the outside world can allow the individual to get in touch with who they are and their belief systems.  Isolation is a means for Melinda to find herself spiritually and emotionally.  It also enables her to have the courage to challenge "It" and to finally open up to Mr. Freeman.  Melinda experiences pain in her isolation. However, Melinda is fundamentally changed from the start of the novel to its end.  Isolating herself from other people is the catalyst for this change.