"The Song of Wandering Aengus" is called a "song" for a reason: it sounds like one. In fact, it's so sing-songy, it could be a children's lullaby. This has a lot to do with the meter and rhyme scheme that's used in the poem. (Check out "Form and Meter" for all those details.) We get plenty of sound echoes, too, though. These effects form a more subtle pattern that catches a reader's ear. Let's look at the second stanza for some examples:
When I had laid it on the floor
I went to blow the fire a-flame,
But something rustled on the floor,
And someone called me by my name:
It had become a glimmering girl
With apple blossom in her hair
Who called me by my name and ran
And faded through the brightening air.
There's a lot going on in terms of sound in this stanza. For starters, there's a ton of alliteration at work here. We get F words (the clean kind) in the first three lines of the stanza ("floor," "fire," and "a-flame"), repeated G words ("glimmering girl"), and B words ("blow," "blossom," "brightening"). As well, we get consonance with the repetition of the M sound in words like "someone," "me," and "my name."
Again, as with the tightness of the form and rhyme scheme, these sound effects are a hard reminder that we're strictly in poetry-land here. Yeats is offering up his poetic take on an Irish myth, and he wants to make it his own. The use of sound effects stamps this as clearly poetic and clearly the work of ol' W.B.