In Emily Dickinson's "I heard a Fly buzz—when I died—," our attention is drawn to sound first by the use of onomatopoeia and the word "buzz."
This word draws our attention perhaps more so than the fly, even though it ("Fly") is capitalized which might initially catch our attention as the reader.
Other words that draw our attention to sound are "stillness," though this word, when combined in the simile "like the Stillness in the Air — Between Heaves of Storm—" gives us the sense that the stillness is charged, not dead.
We also hear sound with the word "breaths," which are also not still, but "charged" with the fear and preparation of impending death ("For that last Onset — when the King / Be witnessed...").
At the end of the poem, the "uncertain stumbling Buzz" may refer to the narrator's uncertain perception of what she hears as her senses fail, or may symbolically represent her uncertainty as she approaches the new, undiscovered country (as described in Hamlet): death.
References to sound abound through the poem, though even the stillness referenced does not necessarily mean quiet or emptiness.