Initially, the narrator's tone is confident but helpless. She clearly expresses her disagreement with the treatments that her husband and brother, both physicians, have recommended for her. She says,
Personally, I disagree with their ideas. Personally, I believe that congenial work, with excitement and change, would do me good.
Her tone is confident; she feels that she knows what would be best for her in her "nervous condition," and not being listened to by her husband and brother is more than upsetting. She tells us that John, her husband, doesn't believe that she is sick, "And what can one do?" she asks. Further, she thinks she knows what treatment would work for her, "But what is one to do?" she asks again. Her constant asking about what one can do makes it seem as though she feels there is not much she can do; she is powerless to effect change in her situation because her husband prevents her from doing everything she wants. This explains why she gets "unreasonably angry with [him] sometimes." She's been made to feel that such anger would be unreasonable, but given her circumstances, it doesn't necessarily feel that way to readers.
As the story progresses, the narrator's disempowered tone turns to one that is much more empowered. Now, rather than be ruled by John's directions, she only pretends to do what he says, like sleep. When he makes her lie down after each meal, she says, "It is a very bad habit I am convinced, for you see I don't sleep. And that cultivates deceit, for I don't tell them I'm awake—O no." With her wallpaper to study, the narrator says that "Life is very much more exciting now than it used to be." She hides her hypothesis about the wallpaper making her better from John because she fears he'd want to remove it. She now says that John "pretends to be very loving," starting to blame him a great deal more for her situation, at least subconsciously.
Finally, she calls the woman in the wallpaper a "poor thing" because she is trapped, and so the narrator "got up and ran to help her. I pulled and she shook, I shook and she pulled, and before morning we had peeled off yards of that paper." In the end, when the narrator comes to believe that she IS the woman she has freed from the paper, she says, "'I've got out at last [...]. And I've pulled off most of the paper, so you can't put me back!'" She now feels totally empowered: first, she was the helpless woman; then she became the woman who would save the woman trapped in the wallpaper; finally, she is the woman who has been freed from the paper. Her disempowerment and imprisonment compelled her to find a way to experience the freedom she lacked, so her mind invented an alternate identity for her to embody: in her own mind, she liberates herself from imprisonment. Ironically, this means...
that she's essentially had a complete mental breakdown, but in her own mind (and thus in her tone), she is powerful and free.