When Gatsby says that Daisy's voice is full of money, he says it so assuredly and means that Daisy belongs in East Egg where the people have never known anything else. Allow me to further extrapolate. Let's begin by looking at the entire text:
"She's got an indiscreet voice," I remarked. "It's full of--" I hesitated.
"Her voice is full of money," he said suddenly.
That was it. I'd never understood it 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. . . . (Fitzgerald 120)
This passage, especially Nick's inner monologue description of it, echoes his very first description of East Egg at the very beginning of the book where he describes the "white palaces of fashionable East Egg" glittering along the bay (5). East Egg, of course, is the land of "old money," meaning that these are people who don't work and have never needed to work. They have grown up with money, . . . and often their great-great-grandfathers had grown up with money as well. The money is "old" because it has been passed down from generation to generation with no worry about it running out. This way of life is incredibly attractive to Gatsby because it something he has never had. Because it is something one is born with, it is something is something that Gatsby never will have, either. The best he can hope for is the "less fashionable" West Egg: the home of "new money," or money that had to be earned. Ah, but this is what makes Daisy so attractive and yet so unattainable. Daisy, in addition to a lover's obsession, also proves to be a monetary obsession as well. In my opinion, Daisy would be the ultimate prize to someone like Gatsby: the acquisition of his love and the only way to achieve the inherited wealth of East Egg.
The comment that Daisy's voice is full of money has so many connotations. To what is Daisy attracted when she visits Gatsby's mansion? She is attracted to his shirts. She cries over them showing her superficiality and Gatsby's superficiality.Why would anyone want someone who cries over shirts?
Another indication of superficiality: Think about when Gatsby's father shows Nick, Gatsby's to do list, the one he made at seventeen. It is supposed to be made emulating Ben Franklin, but again it is superficial in that it's goals have nothing to do with virtue and all to do with acquiring wealth.
And think of the moonlit night when Gatsby looks up at the stars and sees a ladder to heaven. At that moment he can have anything and what he attaches to is very mortal and superficial--Daisy.
Daisy proves the banality of evil when she is able to kill Mabel and yet feel no guilt. In fact she is able to foist the guilt upon Gatsby.
But Daisy represents much more than herself. She represents what the United States has become, something unreal, gilded, but unreal.
If you read the very last passage, you realize that the first Europeans saw a marvelous, green, Eden and they tried to make it into something it could never be because they were all Gatsbys, searching for something they didn't have at home and caused them humiliation because they didn't have that thing, money and all it could purchase, nobility. etc. They didn't realize that if they had taken the land for what it was, they would have had so much more.
No there never was an American dream. America wasn't a dream. It was a completely different entity that no European bothered to see. And Daisy, the corrupt little woman child, is all that they've been able to attain.
Gatsby is trying to say that the way she talks reminds sounds a lot like the way those with money speak. Also the way Daisy is able to discuss expensive things like the expensive fabrics and clothing he bought her. Leading to mean that she sounds almost like every superficial person that comes from money.