What is the whispering and what does it symbolize?
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.