The Eleventh Edition of An Introduction to Literature, by Barnet, Berman, et al, describes "voice" as "persona," defined as:
...literally, a mask; The "I" or speaker of a work, sometimes identified with the authro, but usually better regarded as the voice or mouthpiece created by the author.
In this, "voice" is not a specific person, but the manner by which the author conveys informatio, and is "created by the author." It may be reliable or not, depending upon the author's intent—a distinctly abstract concept.
"Present" voice may speak to the audience in a forthright manner. "Absent" voice may inform in a concealed manner—heard through inferences. However, the "eclipsed" voice may be present, but not clearly expressed—what is being said may be suspect, and the message, convoluted.
In Charlotte Perkins Gilman's "The Yellow Wallpaper," the story is told in the first person, by a woman who is suffering from severe depression brought on by giving birth. Gilman wrote the story from personal experience, describing the "rest-cure" (popular at the time), which robbed a sick mother of company and intellectual diversion—it nearly drove Gliman mad, blaming...
...this experience with driving her 'near the borderline of utter mental ruin.'
She removed herself from the care of Dr. Mitchell. Our protagonist is unable to do so: her physician is her husband. While she secretly disagrees with him...
John is a physician, and perhaps...perhaps that is one reason I do not get well faster.
You see he does not believe I am sick!
And what can one do?
The reader hears the voice of a woman who thinks she is sick, but is at odds with the doctor (John, her husband) who says nothing is wrong with her. She is sure that stimulation will help her; he refuses to let her do anything (so she does it in secret).
As the story continues and the narrator shares her story, the voice is eclipsed—torn between what she believes and what she is told, while also struggling to deal with her illness (e.g., the things moving in the wallpaper). Because she has no one to share her fears and hallucinations with, she has no one to ease her mind and allay her fears. And so, as she spends more time alone, her voice is eclipsed by the madness that is finding its way into her conscious mind. She begins by noticing that something is not right: she believes the fault lies in the house; in truth, it might simply be her state of mind:
...there is something strange about the house--I can feel it.
The narrator is conflicted: she says John is a loving husband, but also says he may be part of the reason she does not get better. Her comments imply he is controlling, not to be reasoned with:
He...hardly lets me stir without special direction.
She abhors the wallpaper that seems to have a life of its own:
…when you follow the lame uncertain curves for a little distance they suddenly commit suicide--plunge off at outrageous angles…
The narrator sees images in the paper: first "bulbous eyeballs." Then she sees the woman:
And it is like a woman stooping down and creeping about behind that pattern.
By the end of the story, the narrator has become that woman, believing she has escaped from the paper where she thinks her husband put her; she creeps along the walls. The voice is now eclipsed because the information relayed is no longer clear or credible to the reader: we can only receive information as she sees it in her mentally deranged state.