Key to consider when answering this question is the way that the voices in the house are impacted by the sudden gift of money that Paul arranges for his mother to receive. You can't help but feel sorry for Paul, as he awaits for the smile of pleasure and the warmth from his mother when she discovers that she has received this surprise "windfall," but instead, as she reads the news, her face only becomes "hardened" and "more expressionless." Note the way that this event impacts the voices in the house. In spite of this money and the wealth and luxury that this gives the family, the voices only worsen in their intensity:
And yet 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!"
This clearly indicates that the voices that haunt the house so powerfully emerge from his mother's unceasing desire for more wealth and greater riches. Even when she has been given money, it is not enough, as is the case with materialism. Whatever we has is never enough, which is the message of this story as Lawrence points out the evils of materialism.