The story has three layers: the author, the narrator, and Marlow, who had the original experience. The voice of Marlow might conflate with that of the narrator, but the author should not be considered the same as they. He adds to the symbolic meaning and crafts the overall themes, although the word choice comes from the narrator / Marlow. The sentence structure also characterizes the narrator and/or Marlow, which is evocative and rhythmic. These sentences in Part I provide an example of imagery and structure and, by means of them, voice: “They were not enemies, they were not criminals, they were nothing earthly now -- nothing but black shadows of disease and starvation, lying confusedly in the greenish gloom. Brought from all the recesses of the coast in all the legality of time contracts, lost in uncongenial surroundings, fed on unfamiliar food, they sickened, became inefficient, and were then allowed to crawl away and rest.” The “they” are the natives, and the details of “black shadows” and “greenish gloom” create mood as well as tone. The parallelism that concludes with “crawl away and rest,” where the language builds to a climax in the final words indicates a person gifted in story telling. This voice is attentive to detail and gives meaning to what he describes through the imagery he uses to describe it.
Well, first of all, he uses English. That's not a smart answer. The story's author wrote in English, but he came from a Polish (or Polish/Russian) family. English was alien to Conrad, and English is alien to Africa. English therefore reveals him as an outsider.
After that, he narrates the story in highly educated language (marking him as a member of civilization, rather than the "savagery" that defined Africa), formal language (pushing him into the abstract and conceptual, but also the judgmental), and high figurative, symbolic language. This last reveals his subconscious and gives the story its tone and deeper meaning.