"'Her voice is full of money,' [Gatsby] said suddenly."
Gatsby says this as an aside to Nick in chapter 7 while a confrontation is heating up between Tom and Daisy. Nick agrees, adding to himself that Daisy's voice had the "charm" of money, "the jingle of it, the cymbal's song" Nick envisions Daisy as "the king's daughter."
Gatsby comments on Daisy's voice as he interrupts Nick, who is presumably going to say something less complimentary about Daisy's voice. Nick has begun by calling Daisy's an "indiscreet" voice as she calls down from an upper window. So, on the surface, Gatsby's observation is probably to forestall whatever comment Nick was about to make, but it also speaks to a deeper truth about why Gatsby is attracted to Daisy. Her voice like "money," along with Nick's addition of the "white palace" and the "king's daughter," speaks to Gatsby being attracted not just to Daisy Fay but also to her house, her white roadster, and her privileged situation. To Gatsby, Daisy is a symbol of not only money, but also of the security that comes with always having had money. Money is in Daisy's very voice, a part of who she is.
Being the insecure individual he was, Gatsby may have been slightly envious as he made this comment about Daisy. He created the fantasy world in which he lived in an effort to make himself appear as someone who came from a wealthy background. He may have been recognizing that it wasn't true for him, but that was an accurate description of Daisy's background.
Gatsby says that Daisy's voice is full of money because Daisy has always lived a life of privilege, and always will. She represents the sophisticated upper crust of the East Egg.
Here is a link to a very good explanation of this: