Just what does a nightingale sound like? According to Coleridge, it sounds like a "musical and swift jug jug" (60). Sure… who doesn't love the sound of a good jug jug?
But that's not what we mean when we talk about the sound of the poem. There's a whole lot of cadence (or, rhythm) in Coleridge's wordplay, and there are a few clever sonic tricks he uses to bring it out.
For example, try reading the following aloud:
By sun or moon-light, to the influxes
Of shapes and sounds and shifting elements
Surrendering his whole spirit, of his song (27-29)
Notice anything? The S sound is repeated quite a few times, at the beginning, middle, and ends of words. That, friends, is called consonance, and it gives a little lyrical lilt to the lines.
Coleridge uses it a ton.
That's not his only trick, though. The poet also sprinkles plenty of exclamation marks throughout the poem. For example:
As if some sudden gale had swept at once
A hundred airy harps! And she hath watched (81-82)
If you read it aloud, you'll notice that the exclamation mark forces you to stop and pause momentarily. Coleridge is using a caesura. Much like the way we speak in real life, the poem starts and stops, building momentum before slowing down a bit.
One other way that this poem makes its sounds pop is through a technique called alliteration. That's a specialized kind of consonance where the beginning sounds of each word are repeated:
'Most musical, most melancholy' bird! (13)
All those M words add to the musicality of the language. Taken with the poem's use of consonance and caesura, we've got a poem that practically sings. And we'd say that's pretty appropriate, given its focus on the nightingale's song.