Gatsby says that Daisy's voice is "full of money" in Chapter 7. He is talking to Nick Carraway when he says this.
I think that the point he is trying to make is that Daisy is a rich person born and bred. She is someone who has never lived and could never live without being rich. When he says this he is, to me, explaining why it is that he worked so hard to get rich. He did it because Daisy has to have a lot of money and he thought that if he were rich, maybe she would love him.
Good fiction writers will try to appeal to all of the reader's senses. F. Scott Fitzgerald makes Daisy's voice an important part of her characterization because the reader will keep "hearing" it, or at least trying to "hear" it every time she is given some dialogue. She never says anything very profound. Many people have asked what Daisy's voice sounds like if it sounds as if it is "full of money." This shows that these readers are using their sense of hearing in their imagination. No doubt every reader will imagine Daisy's voice just a bit differently. Fitzgerald himself must have had a particular pitch and timbre in mind, but it would have been impossible for him to convey to all his readers the precise nuances he himself heard. Fitzgerald has Nick describe his cousin's voice as "low" and "thrilling."
It was the kind of voice that the ear follows up and down, as if each speech is an arrangement of notes that will never be played again....but there was an excitement in her voice that men who had cared for her found difficult to forget: a singing compulsion, a whispered "Listen," a promise that she had done gay, exciting things just a while since and that there were gay, exciting things hovering in the next hour.
While some men might find such a voice fascinating, others might want to call it "cultivated," "practiced," "affected," and a few less complimentary adjectives, such as "spoiled," "self-indulgent," and "phony." No doubt Daisy had plenty of opportunities to try out her tones on boys and men from the time she was sixteen years old (as many young girls will do when they are trying to develop a unique persona). And she probably practiced them in her bedroom at night. The fact that she is a pretty girl, plus the fact that she is a rich girl, combine to make her voice seem all those things Nick ascribes to it; but if she were not pretty and not rich, her voice might not be so hypnotic.