Jay says this in chapter 7 as he, Nick, Jordan, Daisy, and Tom are preparing to drive into the city the day that Daisy, later, hits and kills Myrtle.  From his early days, as shown by the inscription in the old book of Jay's that Jay's father shows to Nick after Jay is killed, Jay Gatsby (James Gatz at that time) wanted to move up in life.  He was determined to make life for himself that was easier than the one his parents had.  Dan Cody cemented that desire when he showed Jay how to live the sumptuous life.  When the group is in New York and the tensions have built to a peak, in the angry confrontation between Jay and Tom, Jay tells Tom that Daisy only married Tom because Jay was poor and Daisy was tired of waiting for him.  It always comes back to money or the lack of it.  To Jay Gatsby, Daisy is his "golden girl"; she is the absolutely perfect woman.  She is young, beautiful, vivacious, and very importantly, she is rich.  When Jay says that Daisy's voice is full of money, he means that she is this perfect woman and that even her voice reflects what is perfect about her.  As Nick explained in his narration following the comment, Daisy's voice reflects the rhythm of wealth and privilege- all that Jay desired.