Speaker for the Dead refers to the central dude, the big kahuna, the one, the only, the star of interstellarness, Andrew Wiggins, a.k.a. Ender, a.k.a. the Xenocide. It's the name of the main character.
But "Speaker for the Dead" isn't just Ender's name; it's a profession, like Priest or Mayor or Minister of Silly Walks. Anyone who tells another's story is, or can be, a speaker for the dead.
Thus, Novinha herself wants to be a speaker for the dead. And she is the perfect one to speak for the alien piggies because, Pip tells her, "You are the one human being who is capable of understanding the alien mind because you are the alien mind…" (1.165). To be the Speaker for the Dead is to be an outcast—Novinha is the alien mind because she is alienated from the Lusitania colony. But it is also to be an interpreter, an understander, one who knows. The Speaker for the Dead is a sad isolated genius who tells the true stories of others.
Speaker for the Dead can also be seen as a reference to Orson Scott Card. After all, Card writes about the Hive Queen just as Ender does, and he tells the piggies's story just as Novinha wants to. Like Ender, he knows what everyone's thinking; and like Ender, he empathizes with them all. And as with Ender, we're supposed to walk away from Card's speaking dazed with the force of his truthfulness.
The title says that this is a book about tellers of powerful stories, and the most powerfullest teller in the room is Card himself, who speaks not just for the dead, but for every character in, and the entire universe of, the book. "Sing us a song, you're the Speaker for the Dead, sing us a song tonight. We're all in the mood for a traumatic but healing eulogy, and you've got us feeling all right."