This novel is pretty depressing until we get to the end. Melinda Sordino finally gets some relief from her brutal ordeal and pieces together her broken life. This relief comes through various forms of speech. "Speech" includes spoken words and body language. It also includes "symbolic speech" – such as writing, art, and other media. For example, at the end of Speak, Melinda finally draws a tree that expresses her emotions.
At the very, very end, Melinda is about to tell Mr. Freeman, her art teacher, her story. Mr. Freeman's willingness to listen to Melinda could also be seen as a form of speech, or at least of communication. By listening to her and showing her that he'll believe whatever she tells him, without judging her negatively, he expresses something as powerful as words or art.
Another reason we consider the ending happy: Andy Evans, who seems to be a serial rapist, is finally revealed for what he truly is. As the graffiti on the bathroom wall of the girls' bathroom shows, Andy has sexually assaulted dozens of girls at Merryweather High, and probably other places, too. Melinda's not the only girl in school who isn't talking. But, she's the first to break the silence. This is important because it makes the dark secret of Andy public knowledge, making it harder for him to strike again. Melinda feels pretty good about the Andy situation after that.
However, Melinda learns the hard way that this is not enough. Official justice (or Lisbeth Salander of The Girl with the Dragon Tattoo) is needed to stop Andy. In the next-to-the last chapter, Andy attacks Melinda and almost rapes her again. Luckily, she fights him off, and there are reliable witnesses to the attack. Although we aren't given details, we can assume that Andy will be prosecuted for his crime. Hopefully he'll be stopped from hurting others in the future.
So far, there's no sequel, but Laurie Halse Anderson, author of Speak, gets requests for a sequel ever day. She wants to write one, but is waiting until "Melinda is ready" ("Here's the thing…" in Speak: 10th Anniversary Edition).