Although they are not entirely different endings, one could easily expand upon what is shown in either the novel or the movie of Speak. In many ways, some readers feel the story is left somewhat incomplete; although Melinda does gain her voice again and uses it to expose Andy as the "beast" that he is, her resolution is only just beginning at the end of the story.
One of the ways you could explore these alternative endings would be to show what happens to various characters as a result of the novel's ending. What happens to Melinda following her freshman year? What does the rest of high school hold for her? How relevant are her art and Mr. Freeman in her on-going healing process? What becomes of Andy? What repercussions occur as the result of his actions? Are they legal repercussions, social consequences, etc.?
I guess I am having a bit of a challenge in reading the question. It seems to me that the fundamental question here is whether we can compose a different ending than what is presented in both mediums. The most evident ending would be that Melinda does not speak and fails to confront Andy. I think that this would borrow a bit from Anderson's latest work, "Wintergirls," where we see a teen girl die from eating disorders. I suppose that an alternative ending would be that Melinda does not externalize her pain in much the same way. Another ending could simply be that Melinda uses her art and her establishment of her special place as a springboard to internal understanding and abandons the social element of her reclamation of voice. This one might not be entirely concurrent with much of Western thought, but it stresses the idea that internal reclamation of voice does not need to be acknowledged in an external element. Essentially, this ending makes the argument that Andy, or "It," is simply that: An object. Melinda's voice is something that is so precious and powerful, so intrinsically good, that it needn't be shared with someone so guttural and base as "it." In this ending, Melinda understands that internal articulation is more important than social externalization. I think that this could be a very powerful ending, but it is not one that is necessarily a part of Western culture.