Daisy's voice is her most captivating quality, remarked upon often.
“Her voice is full of money,” [Gatsby] said suddenly.
That was it.… That was the inexhaustible charm that rose and fell in it, the jingle of it, the cymbal’s song of it.…
This is the most direct and meaningful description of her voice. Taken metaphorically, this description leads to a particular reading of the struggle between Tom and Gatsby. In this reading, Daisy represents money. She is a symbol of wealth.
In the progression of the narrative, Tom starts out with Daisy, just as he began his life with wealth. He was born into it. Gatsby, alternatively, is still working, still building his fortune; still chasing wealth. He is not in possession of Daisy, though he is close and feels that he has a right to her. She is his romantic ideal, his dream.
Reduced to a symbol in this way, Daisy becomes the object of a materialistic struggle that fits into the thematic context of the novel. Ultimately, she is more than just a symbol. She is a well-developed character. Yet the money in her voice allows us to see her (and the story line in which she functions) as carrying a symbolic significance.