How is the symbolism of the whispers in "The Rocking Horse Winner" such an essential part of the story, especially the constant murmuring?

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!