On one level, Gatsby's comment reveals his crassness, the lack of culture behind his facade. To fully understand Gatsby's description of Daisy's voice we must look, in comparison, at how Nick describes Daisy's voice: "I looked back at my cousin, who began to ask me questions in her low, thrilling voice. It was the kind of voice that the ear follows up and down, as if each speech is an arrangement of notes that will never be played again" & "there was an excitement in her voice that men who had cared for her found difficult to forget: a singing compulsion, a whispered “Listen,” a promise that she had done gay, exciting things just a while since and that there were gay, exciting things hovering in the next hour". Both of these references highlight the intense sensitivity of Nick's eyes and ears. They also hint at a form of feeling towards Daisy, his cousin, that goes beyond the mere familial.
Again, later in the first chapter Nick describes Daisy's voice as "glowing and singing", a synaesthetic description merging the impressions of both his eyes and his ears again.
Only a few lines on, Nick again eulogises about Daisy's voice: "a stirring warmth flowed from her, as if her heart was trying to come out to you concealed in one of those breathless, thrilling words" - the stirring here is ambiguous, but the deep emotion perceived by Nick within her words (she is speaking about Nick at the time) suggests that he luxuriates in her attention, almost improperly.
When Daisy and Gatsby are reunited, Nick's first impression is that her voice is "artificial" towards Gatsby, perhaps a manifestation of his jealousy now that her attention has been diverted. Only a few lines on from this Nick again damns her feelings towards Gatsby, and their bond, via his description of her voice - a voice that once glowed - as "her voice as matter-of-fact as it could ever be".
Fascinatingly, Nick changes his tune once her perceives the change in Gatsby - and note the echo of words with Daisy's voice: "He literally glowed". After this impression, Nick alters his description of Daisy's voice from artificiality and triviality to a profound admiration once more: "[h]er throat, full of aching, grieving beauty, told only of her unexpected joy." Nick is clearly struck by their encounter and no longer belittles it. In fact, within a chapter Nick is back to his old, hyperbolic ways: "Daisy began to sing with the music in a husky, rhythmic whisper, bringing out a meaning in each word that it had never had before and would never have again...her voice broke up sweetly, following it, in a way contralto voices have, and each change tipped out a little of her warm human magic upon the air."
On the one hand, Nick happily discusses the fact that Daisy's voice is an affectation, designed to lure men in, on the other he lets himself be lured and his various rich rhapsodies on her voice detail not only its power but also his power as a narrator. Her voice is, if we believe Nick, her most defining feature, and for Gatsby it is "full of money" because Daisy & money are one and the same. Gatsby has conflated his dream of wealth with his dream of Daisy, he chose her rather than the white steps leading to a more rarefied world. Yet her world is "artifical", suggesting any dream of wealth for a poor kid in 1920s America was as elusive, as undefinable, as a beautiful girl's voice.