Sure, "The Voice" is about death, grief, love, ghosts, but the crux of the poem is the speaker's experience with reality. What is real in this poem? What is unreal? Is there an actual voice in the distance, calling out to the speaker? Is there only the wind? Can there be wind and a ghost? Can one hear a ghost calling and not be considered insane? Obviously there are no right or wrong answers to these questions, and that's just fine with us. To get all philosophical on you for a moment, "The Voice" asks about the nature of reality: what's real, what's not. Who is in a position to make these judgment calls? Not the speaker—and not us, either.
The speaker is totally, certifiably bonkers. Sane people just don't hear voices.
Hey! You're nuts for thinking that the speaker is nuts. Grief is intense, man, and causes all of us to experience the world differently.
Death is hanging over this poem like a big ol' Harry Potter-style Dementor, sucking any shred of joy or happiness out of it. The speaker spends the whole poem wallowing in grief—both for the death of his beloved, and for their love which was imperfect while she was living. He's got some confusing emotions going on, and he's having trouble interpreting his reality. His grief is making him just a little unhinged (if not out-and-out insane). Whether or not you believe in ghosts (or, for that matter, the poem believes in ghosts) we can't help but notice that the speaker is having some trouble answering basic questions. Does he hear the voice of his dead beloved? Or the wind? Is this really a difficult question? Death, the poem seems to suggest, troubles the minds of us all.
No ghosts here, gang. The speaker's grief over his wife's death has driven him to madness.
The voice of the woman is totally real—she's a ghost! This poem believes in the afterlife.
"The Voice" has a pretty complicated relationship to the past, probably because there are multiple "pasts" in the poem. There's the distant past, in which the woman was "all" to the speaker (i.e., they were totes in love), and the more recent past, in which the woman was "changed" and she and the speaker were no longer in love. The speaker doesn't just wish that his beloved were alive; he wishes that the woman of the distant-past were alive. So there's this almost double-mourning in the poem—a mourning for the dead woman in general, and a mourning for the "changed" woman she was in the recent past. We get the feeling that the speaker wishes he had worked on his relationship issues before it was too late. (But death comes for us all in the end, whether we've gone through couples therapy or not.)
It's silly for the speaker to mourn for the woman of the distant past. There's really only one woman in this poem.
The speaker is being realistic by mourning for the woman of the distant past. People change, and he didn't like who his beloved had become in recent years. Time is complex, man!