If we accept that (a) a boy gets divinatory powers by riding a rocking horse and (b) a house can whisper, is the story plausible?Is the second of these little more than a metaphor?
It's true a boy is not going to get racing tips by riding a rocking horse he has long outgrown. If we're reading fiction (or watching a movie, many times), there are just some things we have to be willing to accept as possible and move on.
Here's the thing on the house, though--I do believe there is a sense or a vibe which places can have, and, though actual words and sentences may not be discernible, one can know and feel the sense of a place. Living in a household with a limited income or when money is tight, for example, always feels a little desperate. It's in the questions one hears (there's no more whatever-it-is, is there?), the statements being made (don't use it all--that's all there is), and the general tone of the behavior (kind of like cabin fever, being too long in one place without any diversion). Think of a nursing home or a hospital or a funeral home, whose very walls speak of those who are there or who have been there.
In this case, Paul's imagination probably gave him those words, after talking with his mother about luck and money. He's a sensitive kid, and I'm sure those words are somehow his translation of the trouble going on in his house.
Samuel Taylor Coleridge is famous for coining the phrase "the willing suspension of disbelief." What he meant was that the reader must accept the events in a work of fiction or poetry as being true even if they are fantasy or supernatural or even impossible in reality. When we read "The Rocking-Horse Winner," we have to accept just what you propose: that a boy can get racing odds by riding a toy horse and that a house can whisper. When we come to that acceptance, then the story does become plausible for us.
Is the statement that a house can whisper little more than a metaphor? Are you saying that a house is a like a person because it can whisper? That would be a simile (and personification).
To support what linda-allen says, I think we automatically come to literature wanting that famous "willing suspension of disbelief". If we didn't we wouldn't read fiction and would read a biography or history book instead! One of the marks of a great writer is that they are able to achieve this "willing suspension of disbelief" in us without us realising it - we accept it because of their skill in writing. I guess one of the things you might like to think about is are you taken in by Lawrence's skill in this story?