What does Gatsby mean when he says Daisy's voice is "full of money"?

Certainly, I think the interpretation of this line as meaning that Daisy's voice is indiscreet has a great deal of merit. When Gatsby says that her voice is full of money, Nick says,

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. . . .

Daisy's voice seems to be full of qualities associated with having lots of money. She makes no effort to modulate her tone, because she's really never had to be concerned about people not wanting to hear her. She has a certain sense of entitlement because she's been brought up to believe that she is important, and she speaks like it. Further, as someone who's never had to worry about where her next meal is coming from, or how to pay the gas bill, for example, the kinds of concerns she's had have been much less significant than those of the lower classes. She's been relatively untouchable, not subject to the everyday and commonplace cares that most people have, because she's always had money. The "charm" of her voice, the "song of it," seems to convey this.

Before the party leaves Tom's estate and heads into the city, Daisy calls from an upper window to ask if they should bring anything to drink with them, and Gatsby comments to Nick that he cannot say anything in Tom's home. When Nick mentions that Daisy has an indiscreet voice, Gatsby says, "Her voice is full of money" (Fitzgerald, 128). Gatsby is essentially indicating that money and wealth are inherently a part of Daisy's character and a major influence in her life, and this is revealed in her tone of voice. Daisy hails from an affluent family and married Tom Buchanan because of his immense wealth. Her primary goal in life is to maintain her upper-class social status, which is something Gatsby notices in her voice. According to Gatsby, the sound of Daisy's voice reveals her superficial, materialistic personality, and he can tell she is only focused on money. Overall, Gatsby is acknowledging that Daisy's tone of voice is elitist and emphasizes her desire for money and a luxurious life.