The sound we hear running through this poem is always calm and cool. It rustles and whispers, instead of crashing and thundering. It has a smooth, breathy voice, never sharp or harsh or grating. Try saying these lines aloud:
[…] while from all around—
Earth and her waters, and the depths of air—
Comes a still voice— […] (lines 15-17)
They almost force you into a smooth, calm, quiet tone of voice. Just a phrase like "depths of air" is full of soft sounds. All of this reminds us of the sound and feeling of a summer breeze, the kind you might hear while you are lying out in a field with your eyes closed, the bright sun on your face. The breeze might change a little. Sometimes it can be cool and steady, sometimes it’s just an occasional warm gust. Even as the sound and feeling shifts, though, it’s always gentle and calm and full of love and peace. Just like this poem. Even when the topic is sad and grim, the sound is soothing.