Your question has several correct answers.
1. It is an example of a metaphor because Fitzgerald is making an indirect comparison between two seemingly disconnected or unlike objects. Someone's voice is not normally associated with his/her wealth/status; admittedly, we do connect accents, dialect, and vernacular with social classes, but generally, the tone of someone's voice is not compared to his or her social class.
2. The quote is also an example of an idiom in the sense that it must be taken figuratively. Readers cannot extract individual words from the quote, look up their literal meanings, and still correctly infer Fitzgerald's comment about Daisy.
3. Finally, Fitzgerald uses synesthesia within the quote. Synesthesia in a literary sense is the mixing of sensory words (such as "he hit a sour note on the trumpet"). In the above quote from Gatsby, readers normally associated the word "voice" with the sense of sound, but in this case, Fitzgerald associates it with Daisy's intangible/non-sensory characteristics.