Sound structures in poetry are elements that contribute to the aural aspect of a poem. They are what we hear from the poem, whether we or someone else reads a poem aloud, or whether we internally 'hear' the poem by way of silent personal reading.
The rhyme and rhythm of poems are sound structures. Rhyme is readily apparent in some poetry and gives a musicality to a poem along with the metered rhythm of a piece. These elements of a poem help us hear the poem in a different way than we hear prose.
Free verse has rhythm that is less formal - the delineation of line breaks and the choice of words and phrases give free verse its own unique rhythm.
Alliteration is a type of sound structure. Think of the sounds you here in this line from Edgar Allan Poe's The Raven:
Once upon a midnight dreary, while I pondered weak and weary,
The two words that start with the same letter - w - give the first line of the poem a certain aural quality. This is alliteration in poetry.
Assonance is also a sound structure in poetry. Assonance is the repeating of vowel sounds in words that are close to one another in the poem. This gives a certain sound and personality to the poetic line in particular and the entire poem in general.