I am the Poem of the Earth, said the voice of the rain.
In Whitman's poem "The Voice of the Rain" the speaker asks the rain a question: "And who art thou?"
The remainder of the poem provides the rain's answer. The rain, like poetry, exists so as to be eternal. Like good poetry which can withstand the ticking of time, rain exists eternally as well.
I rise impalpable out of the land and he bottomless sea.
As afollower of the Transcendental movement, Whitman embraced an individuals intuition. Here, in regards to intuition, poetry is influenced by Whitman's regard for nature. Whitman's initial question to the rain supports this.
The rain, similar to regarding nature to stimulate a mind and create a poem, resides in both the land and the sea. Many poets get their inspiration from both of these places as well.
All that in them without me were only seeds, latent, unborn;
This line refers to the fact that poets, like the rain, find 'nourishment' in nature which allow their poetry to exist-reflection on nature bring about the poems. The rain acknowledges the fact that a seed cannot grow unless nourished by the rain.
And forever, by day and night, I give back life to my own origin,/ and make pure and beautify it;
This line refers to the fact that both rain and poets 'give back' to all around them. It is the purpose of both nature and poets to purify and beautify the world- in a figurative and literal way. Without the nourishment of the rain and the words of a poet the world would not be as colorful a place.
As for a literal sense of the phrase "music of the earth", think about how the rain sounds. Many poets have used the imagery related to the sound of the rain hitting a metal roof, a window, or a person them self. (Think "Rain on the Roof" by Kinney.)