When Gatsby says that Daisy's voice is full of money, it explains why he has been able to believe that his pursuit of her will be successful. Nick does not identify it initially, but Gatsby does. This causes Nick to reflect on the meaning and significance of Daisy's voice being filled with money:
That was it. I’d never understood before. It was full of money — that was the inexhaustible charm that rose and fell in it, the jingle of it, the cymbals’ song of it. . . . high in a white palace the king’s daughter, the golden girl. . . .
This insight is significant for a couple of reasons. Daisy's voice being full of money reflects her true nature. While Daisy seems trapped or uncertain of what to do or who to become, Gatsby reduces her to her most basic element. Gatsby recognizes who Daisy is. He understands that her love of money, the fact that her voice is "full of it," is how he is able to win her over. He believes that this is the critical element which can allow him to have her as his own. However, it is also this understanding that makes clear her choices will never end up supporting Gatsby. Daisy will always side with money in the form of established wealth. It is for this reason that she will never leave Tom. When Nick describes Gatsby "watching over nothing" at the end of the narrative, it speaks to how empty a pursuit of someone who is only "full of money" really is. Gatsby's statement of how Daisy's voice is full of money and how her entire being can be reduced to money makes his pursuit of her hollow. The one instance of his honest insight is a reflection on the emptiness of his pursuit. Daisy will never leave the established wealth of Tom because her entire being is "full of money," something that Gatsby both understands and simultaneously ignores, believing that she will leave her husband. It is a statement that reveals Gatsby to be profoundly insightful and equally pathetic.