Why does the whispering increase in "The Rocking-Horse Winner"?

In D.H. Lawrence's short story "The Rocking Horse Winner," the voices symbolize the greed of his mother. When Paul enacts a plan intended to satisfy his mother's desire for more money in an effort to silence the whispers and make his mother happy, his plans fail. Instead of satisfying his mother's greed and desire for material things, the money he wins only serves to feed the growing monster of greed. Paul's father has similar flaws, having expensive tastes and very few prospects for making enough money, but Paul focuses his efforts on making his mother happy since her heart is cold toward her children and he is seeking to win her love. 

"There must be more money!"

Yet nobody ever said it aloud. The whisper was everywhere, and therefore no one spoke it. Just as no one ever says: "We are breathing!" in spite of the fact that breath is coming and going all the time.

"I started it for mother. She said she had no luck, because father is unlucky, so I thought if I was lucky, it might stop whispering." "What might stop whispering?" "Our house. I hate our house for whispering." "What does it whisper?" "Why—why"—the boy fidgeted—"why, I don't know. But it's always short of money, you know, uncle." "I know it, son, I know it." "You know people send mother writs, don't you, uncle?" "I'm afraid I do," said the uncle. "And then the house whispers, like people laughing at you behind your back. It's awful, that is! I thought if I was lucky—"

What does the whispering symbolize in "The Rocking-Horse Winner"?

You only get one question, so I'll answer the first regarding "The Rocking-Horse Winner."  The whispering is a personification (a coming to life) of the stress and dissatisfaction found in the house.  The consistent whisper is not literal (real), of course--but the feeling in the house is quite real and tangible to everyone who lives there.  Paul, the son, seems to be the most sensitive to the whisperings of the house. Mother feels she has no luck any more, and more money is the only thing which will change things.  She's wrong, as we discover, but that's what she thinks.

And so the house came to be haunted by the unspoken phrase: There must be more money! There must be more money!...  Yet nobody ever said it aloud. The whisper was everywhere, and therefore no one spoke it. Just as no one ever says: "We are breathing!" in spite of the fact that breath is coming and going all the time.

Though this is a family who apparently has plenty of money (as evidenced by the fact that they have servants and stables and expensive gifts and other such extravagances), there's a constant hunger for more.  This desperation is what is being whispered by the house.  Once money does come into the mother's hands (thanks to her son, of course), the voices don't subside; in fact, they get louder and more demanding. 

...the voices in the house, behind the sprays of mimosa and almond-blossom, and from under the piles of iridescent cushions, simply trilled and screamed in a sort of ecstasy: "There must be more money! Oh-h-h; there must be more money. Oh, now, now-w! Now-w-w - there must be more money! - more than ever! More than ever!"

Your question is what does the whispering symbolize.  The answer, I think, is the discontent and greed of Paul's mother.  She is never satisfied, and the the feeling in the house (as represented by the whispers) is indicative of those two things.  It's similar to walking into a room in which there is tension or guilt or whatever other emotion you might think of, and recognizing that something is going on, even without knowing any of the specifics.  It's just there, as if the room is whispering.  Same thing here.  The house is speaking what's in the heart and mind of Paul's mother, and it's what eventually kills her son.

Does the house really whisper in "The Rocking-Horse Winner," and why can only the children and the toy hear it?

In "The Rocking-Horse Winner," the house is personified at the very beginning of the story as having human feelings "as they [the family] felt always an anxiety in the house" (paragraph 3). The house's anxiety grows larger and larger "and so the house came to be haunted by the unspoken phrase: There must be more money!" (paragraph 5). Just as the house is personified as having anxiety, Lawrence also uses personification in making the house speak. Interestingly enough, it only speaks to the children and the toys as both parents appear unconcerned as to the price their children are paying for their greed. The parent's unhappy, materialistic life has pervaded the entire house, and the children feel this misery intently.

As the story progresses, when Paul gives his mother the entire five thousand pounds, the house begins to scream and is described as "mad" when Paul's mother spends the money as quickly as she can. This personification and Paul's terror at the house's reaction foreshadow his death at the end. Therefore, since personification is a literary device that can give human characteristics to nonhuman subjects, the house can whisper.

The story is written in a fairy tale or fable style.  Just look at the first paragraph where we are introduced to the characters and conflict.  If you view it as a modern day fairy tale, which magical elements, then the house could certainly whisper to the kids. 

One reason the children are only able to hear it is because the mother is oblivious to their needs and the house's needs.  She is living beyond her means, so there is always a need for money.  However, she does little about it.  When Paul is able to earn money for her, she doesn't save it.  Instead she burns through it, causing the house to call louder than ever for more money. 

The fact, that her mother cannot hear the whispers is one reason Paul is driven to ride the rocking horse in order to compensate for the money his mother desires and the money his father isn't luck enough to bring in.

What is the significance of the whispers' symbolism in "The Rocking-Horse Winner"?

The whispers in the house, and the constant murmuring symbolizes greed and materialism's insatiable appetite, and how gaining love through money doesn't work.  Note how even after Paul wins his mother five-thousand pounds, the house's whisperings intensified; they

"simply trilled and screamed in a sort of ecstasy: 'There must be more money ! Oh-h-h; there must be more money.  Oh, now, now-w ! Now-w-w --- there must be more money ! --- more than ever ! More than ever !'"

So, even when Paul got more money, which is what the voices were wanting, it was never enough.  This is true of any person seeking love, affirmation, or closeness through money.  It doesn't work.  It is never enough, because it is like filling a sieve with water; the substance runs through, leaving the vessel empty once more.  Paul thinks that earning money will earn him his mother's love.  It is an obsession, which is why the voices, which are symbolic of his quest, murmur constantly

"behind the sprays of mimosa and almond blossom, and from under the piles of iridescent cushions,"

everywhere.  It is behind all of his motivations, ever-present.  Until Paul realizes that he will not win his mother's love through "luck" or money, the voices will always be there.  I hope that those thoughts help; I also provided links below to further discussions of theme in the story.  Good luck!