explain on your own words "I heard all things in the heaven and in the earth. I heard many things in the underworld."

This question seems to be open for interpretations, and as such I can offer you my understanding of this quote.

This quote is form Edgar Allan Po's short story The Tell-Tale Heart and, with taking that context into consideration, it can be interpreted as an reinforcement of the idea (later on fact) that the narrator is insane. Narrator of this story is paranoid and maybe schizophrenic and he hears voices in his head that are probably both good and bad which is why he says that he heard all things on heaven, earth and in underworld. Even though he puts an emphasis that disease didn't make him mad, but that it has sharpened his sense of hearing, we can clearly see later on in the story that these voices were due his disease and not his supernatural sense of hearing.

Without this particular context this quote could be interpreted as the words of the prophet as he has the connection with heaven, earth and hell. Using prophets to convey specific idea was common in religious, ancient and Romantic literature.