Great question! Clearly the story suggests that it is the mother's greed and desire for greater material wealth that creates the voices that echo round the house and haunt Paul so badly - "There must be more money!" However, it is interesting that when Paul does gain money through his rocking horse and organises to give it to his mother, she is far from satisfied. Note how she responds:
He knew the lawyer's letter. As his mother read it, her face hardened and became more expressionless. Then a cold, determined look came on her mouth. She hid the letter under the pile of others, and said not a word about it.
Later we are told that she asks the lawyer for the entire sum straight away rather than agreeing to be given it bit by bit. Then a strange incident is reported about the voices. In spite of the large sum that his mother has been given, the voices become louder:
And yet the voices in the house, beyond 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! Ohhh; there must be more money. Oh, now, nowww! Nowww - there must be more money! - more than ever! More than ever!"
Thus it is clear that in response to this sudden gift, the mother's greed and desire for more is worse than ever. Lawrence is clearly showing how materialism is a vicious cycle, where when you gain more you simply want more, which of course in this story results in the death of Paul as he is driven to ever-greater efforts to find out winners.