The voice coming from above, from the clouds, may not be God’s voice, but it’s a spiritual voice nonetheless. The very fact that the speaker describes the noise that the thunder makes as a “voice” suggests that there is something (or someone) speaking to us through the thunder. There’s a spiritual presence in the clouds, and the thunder is its expression.
Again and again it sounds, The voice that beautifies the land (5-6)
In telling us that the voice of thunder sounds “again and again,” the speaker suggests that we can’t escape the power of this voice. And this power is itself a reflection of the spiritual power of nature, which we can’t ignore.
The voice below, The voice of the grasshopper (8-9)
Here, again describing the noise that a grasshopper makes in terms of a “voice,” the speaker suggests that there’s a spiritual presence that speaks to us through the noises that the tiniest insects make. We may not understand what the grasshoppers are telling us (unless, you know, we have magical powers that can make us understand grasshopper language) but we understand enough to know that something else—something bigger than them—is speaking through them.