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.