"To Helen" is a love poem. It's got all the classic sounds of one, from the enumeration of attributes ("thy beauty," "thy […] hair," "thy […] face," "thy […] airs") to grandiose assertions about the effect of Helen's beauty on the speaker (the whole bit about being a "weary, way-worn, wanderer" comes to mind). There are also other ways in which this poem sounds like love, too.
Consider the various forms of repetition in the poem. First, there's alliteration (the repetition of the first letter): "weary way-worn wanderer" (4), "hyacinth hair" (7), and "glory…Greece" (9). Then there's assonance (the repetition of vowel sounds): "hyacinth" and "Naiad" (7, 8), "long wont" (6), "agate lamp" (13), "hair" and "air" (7,8). Then of course you have all the end rhymes, which we talk all about in our "Form and Meter" section. The effect of all these sound echoes in the poem is an impression that, on a sonic level, everything seems to fit together. Really, that kind of unity shouldn't be too surprising in a love poem. Just read it out loud and tell us it doesn't seem like the sounds of the poem are in love with themselves, in the same way the speaker is in love with Helen.
The poem's final stanza is also crucial to the poem's soundscape. Notice that he keeps interjecting, and each time he sounds like he's stunned, moved, or totally floored: "Lo" (11), "How statue-like" (12), and that swooning "Ah." That's what people either think or say to themselves when they see something incredible, something that they love. These are all sounds of awe, reminding our ears just how super-incredible our speaker thinks Helen is.