How does Bradbury use diction to convey the tone in the exposition of "There Will Come Soft Rains"?

The diction in the exposition of "There Will Come Soft Rains" creates an immediate contrast between the technology running the house and the ominous emptiness within it.
On one hand, the voice of the house is pleasant and helpful. It "sings" its reminders and even presents them in an upbeat and playful voice: "Tick-tock, seven o'clock, time to get up, time to get up, seven o'clock!" It provides a loving reminder of an anniversary and friendly reminders to pay various bills. The sing-song reminders continue as the voice of the house provides a weather update: "Rain, rain, go away; umbrellas, raincoats for today..." The word choice as used to characterize the house as engaging, cheerful, and helpful.
In contrast, the diction used to characterize the house itself is quite different. The house lays empty. The clock "[repeats] its sounds into the emptiness." The breakfast stove "hisses" and "ejects" toast, eggs, bacon, coffee, and milk. It is noted that there are no slamming doors and no heels running across carpets, and only the rain creates sounds in a completely desolate house.
This contrast based on carefully selected diction creates a striking image of all that is wrong. Technology was created for human use, and without it, everything is rendered meaningless. The technology of the house makes great effort to continue in its mission to serve, but its purpose is now gone. Bradbury uses italics to further show how diction creates a sharp division between the intended function and the actual reality of this technologically-driven house.

Diction is the author's style of writing, the language they choose to use. Exposition is the background a story needs to supply to make sense.
In "There Will Come Soft Rains," Bradbury uses lyrical or poetic diction to convey a tone of poignancy (sadness) and futility as the house goes about the daily tasks meant to make life easy for its owners. The house has no way to know that the family it serves has been annihilated in what appears to have been a nuclear war. Bradbury relies on imagery, description that appeals to the five senses of sight, hearing, sound, taste, and smell, to convey the tone of sadness that permeates the story. For example, we learn that

The garden sprinklers whirled up in golden founts, filling the soft morning air with scatterings of brightness . . .

We can picture the water, described in gentle, lyrical, positive terms: "golden" and "bright" in the soft air. This makes it all the sadder when we learn, through another set of images, that the family is dead:

The entire west face of the house was black, save for five places. Here the silhouette in paint of a man mowing a lawn. Here, as in a photograph, a woman bent to pick flowers. Still farther over, their images burned on wood in one titanic instant, a small boy, hands flung into the air; higher up, the image of a thrown ball, and opposite him a girl, hands raised to catch a ball which never came down.

In the quote above, we can see exactly the kind of ordinary activities the family was engaged in when the nuclear bomb hit. This description brings to life the tragedy of their surprise deaths. Bradbury doesn't tell us something bad has happened: he shows it to us.