In Joseph Conrad's "Heart of Darkness," the narrator Marlow is making his way up the river Congo into the depths of the Company's territory to retrieve its star agent, Mr. Kurtz, who has fallen ill. Kurtz has been an unseen but constant presence in the story since Marlow joined the Company. Everyone has an opinion about the man, for good or ill, even those who have never met him. Marlow becomes fascinated by Kurtz and his fascination intensifies the further upriver he travels. Kurtz is at the most remote ivory station in the territory, so reaching him is very difficult. The closer Marlow gets to Kurtz's station, the more the mythos surrounding Kurtz deepens, as what were vague rumors on the coast are now solidifying into unsettling testimonials from people who know or have been influenced by him. The Russian traveler is one such person, who evangelizes for Kurtz as if Kurtz were some kind of divine figure:

[The Russian traveler] rattled away at such a rate he quite overwhelmed me. He seemed to be trying to make up for lots of silence, and actually hinted, laughing, that such was the case. "Don't you talk with Mr. Kurtz?" I said. 'You don't talk with that man—you listen to him,' he exclaimed with severe exaltation.

Marlow doesn't know what to make of any of this. The facts about Kurtz are so minimal that Marlow can only imagine what he must be like, and in his imagination, he says,

The man presented himself as a voice [...] of all his gifts the one that stood out preeminently, that carried with it a sense of real presence, was his ability to talk, his words[.]

Marlow is determined to speak with Kurtz when they reach the station, to try to understand what hold this man has over others, and he is afraid that due to the various delays in his journey, Kurtz will die before he can reach him.

I was cut to the quick at the idea of having lost the inestimable privilege of listening to the gifted Kurtz. Of course I was wrong. The privilege was waiting for me. Oh, yes, I heard more than enough. And I was right, too. A voice. He was very little more than a voice.

Kurtz's being has been eroded by his time in the wilderness; he has lost his hair, his weight, his sense of morality, and most of his mind. What he has not lost is his way with words, with which he dominates all those in his presence. The faster his strength and sanity fades, the stronger his voice becomes, and it seems to Marlow to be Kurtz's last remaining connection to the world, or rather, with himself—his true self, the one who originally came upriver full of fine notions about civilizing the natives and "[exerting] a power for good practically unbounded." Kurtz's true self has long since been devoured by his "monstrous passions," which he now, on his deathbed, struggles to convince himself were worth gratifying. His voice is his only defense against the truth of what he has become, and he uses it to retain control of his world.

Kurtz discoursed. A voice! a voice! It rang deep to the very last. It survived his strength to hide in the magnificent folds of eloquence the barren darkness of his heart.

Kurtz commands, and others obey; he declaims, and others listen. His voice and his words are his power, and in his corner of the world, his power seems to be absolute. And yet, Marlow discovers, this is an illusion, and Kurtz is entirely at the mercy of his own savage appetites:

Everything belonged to him—but that was a trifle. The thing was to know what he belonged to, how many powers of darkness claimed him for their own.

[...]

I think [the wilderness] had whispered to him things about himself which he did not know, things of which he had no conception till he took counsel with this great solitude—and the whisper had proved irresistibly fascinating. It echoed loudly within him because he was hollow at the core....

All that remains of Kurtz, unclaimed by the "powers of darkness," is his voice. As he succumbs to his illness, he talks incessantly about his life, his career, and his grand ideas, in an effort to stave off the revelation that all of these have come to naught. He cannot bring himself to truly face his evil, and wants to convince Marlow that all his depraved actions were aimed towards some higher goal:

"I was on the threshold of great things," he pleaded, in a voice of longing, with a wistfulness of tone that made my blood run cold.

This illusion that Kurtz has so strenuously maintained fails him in the final moments of his life, and his voice, which he has used to justify, defend, and explain his actions, gives way at last to the truth in an anguished cry—"The horror! The horror!"—as he comes face to face with what he has done.

Marlow's first and last impressions of Kurtz are of his voice, and in speaking of Kurtz's voice, Marlow is speaking more broadly of Kurtz himself, for all he ever knew of the man is what he heard from others and what Kurtz himself told him. Kurtz's vaunted eloquence, so much on display in his pamphlet and his interactions with others, is reduced by the end of his life to a short gasp. In the same way, the brilliant, gifted man who left his Intended and went into the wilderness has been reduced by that wilderness to a gaunt invalid, his intelligence unhinged and his nobility debased.

As Kurtz lies dying from his illness, he gives long speeches about his life and his opinions. His body can no longer perform his will, but his best talent -- his eloquent speaking style -- remains almost to the last.

Kurtz discoursed. A voice! a voice! It rang deep to the very last. It survived his strength to hide in the magnificent folds of eloquence the barren darkness of his heart.
(Conrad, Heart of Darkness, eNotes eText)

Marlow sees that Kurtz is so damaged by the jungle and his actions that his body has given out; Kurtz has nothing left to offer the world except the contents of his mind, which he is slowly losing. In an effort to leave one last mark on the world, he speaks constantly, letting loose every aspect of his mind regardless of the opinion of others. Kurtz's voice is the last aspect of himself that has survived his descent into the "heart of darkness," a descent that has even overpowered his own inner heart and soul.