This first line of the poem is already giving us a sense of the speaker’s awe and amazement. It’s a line that ends with an exclamation mark. That exclamation mark suggests that the speaker is pretty excited, or awe-struck, by this voice he’s hearing.
The voice of thunder Within the dark cloud (3-4)
Even though this is a pretty simple description of thunder and dark clouds, the imagery is still pretty powerful. Thunder and dark clouds are a spectacular sight (and sound). So we can assume that the speaker is pretty amazed by the vision and sound of thunder and clouds. Why else would he be including a description of them in the song?
The voice of the grasshopper Among the plants (9-10)
The speaker is just talking about an itty-bitty grasshopper in the grass here. And yet, by mentioning this grasshopper after the thunder, and by speaking about it in the same terms as he did about the thunder, the speaker suggests that he is as awed and amazed by the little things in the grass as he is by the grand things up in the clouds.