The general tone of the poem is one of wistfulness, a vague sense of longing to see the trees—and to hear the gentle breeze blowing through them—in their natural environment. If not for the traffic and the trains, the trees would be heard as if in a country lane, and they'd make a sound every bit as thin and sweet.
And that leads us on to the overarching theme of the poem, which one could argue is the indifference of nature to its immediate surroundings. Whether lining a country lane or a bustling city street, the trees will always make the same sound whenever a soft breeze brushes against its leaves. It's just that those who live in the city can't always hear it above the din of traffic. But it's there all the same, as the speaker acknowledges in the final lines:
Oh, little leaves that are so dumb
Against the shrieking city air,
I watch you when the wind has come,—
I know what sound is there.