The lights grow brighter as the earth lurches away from the sun, and now the orchestra is playing yellow cocktail music, and the opera of voices pitches a key higher.
In this passage, Fitzgerald is explaining that as the night goes on, people are getting more and more intoxicated. Thus making everything seem different. (i.e the lights are brighter, and the pitches are higher)
To help improve the meaning of these lyrics, visit “The Great Gatsby (Chapter III)” by F. Scott Fitzgerald and leave a comment on the lyrics box