Any poem that has 495 lines is sure to have a lot going on, sound-wise. Shelley had plenty of tricks up his sleeve to give this elegy a bit of sonic boost. Let's go in for a closer look.
Got your tongue in a twist? Shelley uses lots of alliteration in this poem, which is the repetition of beginning consonant sounds (think "Susie sells seashells by the seashore"). This gives the poem an internal flow and energy, not unlike the beats used in rap music. For example:
Forget the Past, his fate and fame shall be (8)
The repetition of the F sounds—which force us to push the air between our upper teeth and bottom lip—adds a force and sense of conviction to this line. Want another example?
Month follow month with woe, and year wake year to sorrow. (189)
That one's a doozy. The M, W and Y sounds combined make for one literally luscious line (see what we did there?), strengthening the impact of this line by keeping the sounds tight and unified. Keep your eyes peeled as you read; alliteration is one of Shelley's favorite tricks.
We know that Adonais has a pretty specific rhyme scheme (see "Form and Meter" for more), but Shelley doesn't always play by the rules. Several of the rhymes in the poem are slant rhymes, sometimes known as "half-rhymes." They get close to a full rhyme, but don't quite make it the whole way. For example:
Where wert thou, mighty Mother, when he lay,
When thy Son lay, pierc'd by the shaft which flies
In darkness? where was lorn Urania (10-12)
In this stanza, "lay" and "Urania" are meant to rhyme, but they just… don't quite get there. This is an example of slant rhyme. In an elegy, slant rhyme is a pretty appropriate technique. Think about it: do you want everything tied up in a nice, neat bow of rhyming echoes when you're talking about devastating loss? We didn't think so. Slant rhyme, then, is a subtle way to show that something is off, not quite right (or rhymed).