Generally speaking, the more vowel sounds, the more mellifluous and melodious the language sounds.  Vowels take longer to pronounce than consonants do, and long vowels (the "a" sound in way, the "e" sound in "we", the "i" sound in ice, the "o" sound in drove, the "u" sound in super) take even longer to pronounce than short vowels (the "a" sound in cat, the "e" sound in wet, the "i" sound in wit, the "o" sound in bought, the "u" sound in up).  Tennyson opens with the lines, "On either side the river lie / Long fields of barley and of rye," you can hear the long "i" assonance in the words either, side, lie, and rye, as well as the long "e" assonance in fields and barley (also, either, depending on how you pronounce the word: with a long "i" or a long "e" in the beginning).  In starting the poem with this assonance, especially with long vowels, Tennyson establishes a slower pace, a pace that mimics the flowing stream he references in the second stanza and makes it clear that we are going to hear a story.  We get the sense that we should settle in for the narrative as its opening sounds sort of lull us, producing an almost hypnotic effect (especially when combined with the end rhyme, which naturally employs assonance).  

Toward the end of Part I, Tennyson describes a shallop, a light sailboat, saying, "The shallop flitteth silken sail'd, / Skimming down to Camelot."  We can see here how the sounds he employs—particularly the alliteration of the words beginning with the "s" sound—are meant to call up or remind us of the sounds one might hear if one were actually there, in the poem.  The words silken, sail'd, and skimming begin with the "s" sound that is often used to remind us of the sound water makes when it moves gently.  Thus, we can see how Tennyson uses sound and musical devices to control the pace and mood of the poem, as well as to add to the imagery it employs by duplicating the sounds associated with the things he describes.

There are many examples of assonance, the repetition of vowel sounds, and alliteration, the repetition of consonant sounds, in "The Lady of Shallot" by Alfred, Lord Tennyson. Tennyson uses these literary devices in several ways. A typical example of alliteration occurs in the line:

Willows whiten, aspens quiver,

The alliteration of two initial consonants before the caesura in a four stress line evokes the sound of Anglo-Saxon strong stress alliterative verse, giving a sonic echo of the medieval atmosphere Tennyson is attempting to create.

For assonance, Tennyson often uses a long "i" sound, such as "silent isle" and "silent night" in the poem. This serves to slow the pace of the poem, emphasizes the languor and separation from the world of "getting and spending", and to a degree monotony, of the Lady's life.