How does the author show that different electronic devices are speaking in "There Will Come Soft Rains"?

The author uses italics to show what the clock is saying, and regular quotation marks for the other electronic voices.  The house is fully automated, and the different devices talk, trying to communicate with the people who are no longer there.  The house never realizes they are gone.

The first device we hear is the clock.

In the living room the voice-clock sang, Tick-tock, seven get up, seven o'clock! as if it were afraid that nobody would. The morning house lay empty.

The clock speaks repeatedly.  It sings out the time throughout the story, which is very chronologically-based.  We experience a day in the life of the automated house on its last day before it burns down.  The clock continually reminds the people what time it is, even though the people are dead, turned into spots of paint.  The house does not realize this.

The clock is not the only voice in the house though.

"Today is August 4, 2026," said a second voice from the kitchen ceiling, "in the city of Allendale, California." It repeated the date three times for memory's sake. "Today is Mr. Featherstone's birthday. Today is the anniversary of Tilita's marriage. Insurance is payable, as are the water, gas, and light bills."

The house seems to do everything for the people.  It makes meals, reminds them of appointments and due dates, and cleans itself.  That’s pretty cool, except that the house cannot exist forever without the people.  When no one comes home, the house is on its own to protect itself.

Eventually, the house is destroyed in a fire.  The house is aware of the fire too.

"Fire!" screamed a voice. The house lights flashed, water pumps shot water from the ceilings. But the solvent spread on the linoleum, licking, eating, under the kitchen door, while the voices took it up in chorus: "Fire, fire, fire!"

Presumably, if there were people there they would be warned by the voice and act.  Since there are no people, it just seems as if the house is crying for its life.  The house has fire-fighting features, but they are ineffective.