Right from the beginning, we realize that the narrator well-spoken but is also trying to convince the reader (and himself!) that he is sane even though he apparently is being accused of being otherwise. By mentioning the imagined sounds from hell, we realize that the narrator is indeed "unwell" and only evil things will be revealed in the story, otherwise the narrator's obsession with being deemed as sane would most likely not be there. In addition, to want to harm a person merely based on his eye, which is perceived as hideous (he claims he loved the man, doesn't desire his gold, and has never been wronged by him) is completely illogical and confirms our suspicion of his mental instability.
The Tell-Tale Heart begins with language that not only draws the reader into the story, but also lets you know that this narrator - clearly, a mentally unbalanced person - is telling you about something that has already happened. So, in a sense, he is not foreshadowing, but rather flashing back to an event in his past. But language like, "I heard all things in the heaven and in the earth. I heard many things in hell," definitely lets the reader know that this is not going to be some nice, happy story - Poe sets the stage for a sinister tale of madness. Check out the link below as well as other good articles at eNotes on Poe!