In The Great Gatsby, Gatsby views Daisy as a material object, like money, and therefore attainable and kept as such. Like the green light at the end of her dock, Daisy's voice is like the American Dream which lures and beckons males to achieve it at any cost.
By saying Daisy's voice is full of money is ironic in two ways. First, Fitzgerald is suggesting that Daisy's language is that of the rich in America: Daisy speaks for rich women (housewives). The irony, of course, is that she doesn't say much. She is full of gossip, pettiness, and frivolity. She calls herself and her daughter, all women in fact, "hopeless little fools."
Secondly, Daisy's voice attracts money. She has two men competing over her, and she bounces from party to party in mansions in East Egg, West Egg, New York City. So, Fitzgerald is suggesting that women are not prized for their voices at all; rather, they are status symbols like cars, shown off by men to show other men that they've achieved wealth in America.
The phrase is designed to surprise readers. We don't generally associate or describe female voices with references to money. For Fitzgerald, Gatsby's bit of dialogue operates metaphorically. Gatsby's odd comment about Daisy's voice also reminds readers that Daisy is a symbol of money. That Gatsby considers everything about Daisy to be an aspect of money and a wealthy lifestyle reminds readers to look closely at the nature of Gatsby's love for Daisy. What does Gatsby love about Daisy? What sort of love story is this? When Gatsby compares Daisy's voice not to music but to money, Fitzgerald reminds readers that, for Gatsby, Daisy is a symbol of wealth. Fitzgerald uses figurative language that suggests that Daisy is merely a symbol in order to surprise and thereby warn readers about the true nature of Gatsby's love for Daisy. Gatsby's blunt valuation of Daisy underlines the striving, ambitious, materialistic, and status-seeking nature of Gatsby's love for Daisy.
I would actually call it a rich description or perhaps a figure of speech. It is not a traditional metaphor, although it functions like a slanted metaphor to an extent. On the other hand, it could be seen as a symbol as money is a recurring symbol (or the air of having money) throughout the work. I would argue it as a figure of speech, but if I had to choose I would choose metaphor. A metaphor is a comparison of two things (nouns) that are not the same. In this case, we are comparing her voice to money in a slanted way. A metaphor IS a figure of speech, so the definition loosely fits. For a definition of metaphor, see here:
A symbol, on the other hand, is something that represents something else entirely. As her voice being "full of money" represents an attitude and a character who is defined by money, the definition doesn't really fit.