What a world of merriment their melody foretells! (line 3)
The fact that the sound of the bells is described as a "melody" seems like an important part of the opening of this poem. We're not talking about some sort of flat "ding, ding, ding." The song of the bells is full of life and cheerfulness. Of course bells on a sleigh can't really play a song with an actual melody. The point here is more to pay attention to the ways that notes and sounds and rhythms can affect us. That's exactly what Poe is exploring with the words and the rhythms of "The Bells."
In a sort of Runic rhyme (line 10)
The speaker doesn't just compare the sound of the bells to music. He also mixes in comparisons to poetry and literature. The phrase "Runic rhyme" is a little bit mysterious. We use to the word "rhyme" to describe little poems (like nursery rhymes). So a Runic rhyme could be some ancient, strange poetry. Plus, those two words just kind of roll off the tongue, don't they? That play with sound is a big part of this line's impact.
What a liquid ditty floats (line 22)
Can't you just see this song floating through the air? You know when a person sings in a cartoon and you can sometimes see the notes flowing out into the world? That's exactly how we picture this song, like a delicate cloud wisping out of the bells.