In the opening of chapter nine of Beloved, the following words appear: "words whispered in the keeping room." Here, Sethe is remembering a time prior to Paul D coming out to the porch. For her, the words whispered had kept those who were no longer around her around. The narrator tells the reader that the voices made Sethe able to feel Howard, Buglar, and her husband a part of her life. Without these whispered words, none of them would be a part of her life. Instead, she would only be able to see parts of Howard and Buglar (in the trees) and not even her husband's shadow would be a part of her life.
While one can only assume that the words were actually whispered, a more justifiable idea would be that the words come from her own memories which keep Buglar, Howard, and her husband alive for her.