wife. Since the poem is written in the first person, the Duke only refers to himself as "I."

Since the narrator's wife was a Duchess, we know that he is a Duke. The single word, Ferrarra, immediately following the title of the poem suggests that he is the Duke of Ferrarra, an Italian city northeast of Bologna. The Duchy itself was created in 757 AD, and thus the reference to a "nine-hundred-years-old name" refers to the title of Duke of Ferrarra, which gave his wife the title of Duchess of Ferrarra.

The poet does not assign personal names to either the Duke or Duchess, but just refers to them by their titles. 

The tone in literature is the attitude of the writer. The tone of "My Last Duchess" is ironic: Browning insures that the Duke, in justifying his murder of his young former duchess, manages to unwittingly paint himself as a sociopath. The duke does the opposite of what he intends, which is what irony is. Instead of inviting sympathy, we recoil from him in horror.

However, the Duke's voice or speech patterns in the poem are completely sincere. Browning conveys this voice through his writing style, which involves having the Duke recounts the details of incidents with the duchess that upset him, such as her equal appreciation of him and

The dropping of the daylight in the West,

The bough of cherries some officious fool

Broke in the orchard for her

Browning also conveys the Duke's feelings with such characteristics of Browning's style as a set ABAB rhyme scheme, dramatic monologue (letting the Duke tell his story to a silent party) and use of narrative or storytelling.

I think the two terms have become conflated, which is to say they may be used interchangeably. To delineate a difference is to split hairs, I think. And it is akin to snobbery. I make no distinction between the two.

The speaker's voice or tone in "My Last Duchess" is one of arrogance, jealousy, misogyny, and patriarchy. He sees women as objects, like the very painting he illustrates to the messenger. He prides himself on his 900 year-old name.

Reputation, as you know, was once tied to one's identity and soul. Cassio, in Othello, says, "O! I have lost my reputation. I have lost the immortal part of myself, and what remains is bestial." The duke's last duchess did indeed insult his reputation by glaring and smiling at everyone in the same manner. How dare she be equitably nice to everyone? In his eyes, she should reserve her glares and smiles for him alone, privately.

That he thinks she might have cheated on him with the painter does not matter. Whatever reason he invents to condone his attitude toward her is inconsequential. He has reified her, turned her into property, and her death is nothing but a dismissal to him. And his words to the messenger are a warning to the next duchess.