Instances in Speak where Melinda begins to find her voice involve her actively defining herself against the world around her.
Recognizing the inauthentic nature of people she used to consider as friends is one way Melinda begins to find her voice. Melinda understands that friends like Rachel have moved on from her. This forces her to find her voice because she understands that there is no "there" there. She lacks any real connection to people from her past. Rather, as Melinda's freshman year begins, she understands that she is on her own and that there not much she can do to avert that reality, forcing her to find her voice.
Another example of this emergence of voice can be seen at the pep rally. It is the first time that Melinda acknowledges much of the pain she experienced that summer night. When she screams in her hands, it is a moment where her voice develops on two levels. There is the audibility of her voice. This is something that had not been seen up to that point. It is also an instant where she recognizes that she is fundamentally different from those around her. Melinda's experience at the pep rally helps her to understand that she is not going to be like everyone else. Her identity formation is going to take place on a different track than others. Melinda comes to understand this through her experience at the pep rally.
I think that another instance where Melinda's voice emanates is in her criticism of the high school institution. Melinda comes to understand how it is not predicated upon what it preaches. For Melinda, this is an example of voice because she is able to point out limitations in reality. She does not blame herself and does not retreat into a realm of silent desperation. Melinda's voice develops when she is able to articulate the deception in statements such as "You will use algebra in your adult lives" or "We are here to help you" and "These will be the years you look back on fondly." In being able to indict the institution, Melinda affirms a specific strength in defining herself with a different type of voice.