The refrain of this line that the poem opens with suggests that time moves in a cyclical way. The voice of thunder and of the grasshopper beautify the land over and over again.
The voice of thunder Within the dark cloud Again and again it sounds (3-5)
These lines again emphasize the idea of time as cyclical. The “voice of thunder” sounds “again and again.” It’s a repeated sound. This suggests that things don’t happen once, they happen over and over again in nature.
The voice of the grasshopper Among the plants Again and again it sounds (9-11)
The voice of the grasshopper, like the voice of thunder, sounds “again and again.” So here, we’re also getting this idea that things recur in nature, which means that time moves in circles. The line “again and again it sounds” is itself repeated from the first stanza. So by using repetition in this way, the speaker dramatizes the repeated movement of time.