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.
What a world of solemn thought their monody compels! (line 72)
Here the speaker is offering us another comparison between the music of the bells and other kinds of art. A "monody" can be many things – a song sung by one person, a funeral poem, or a sad song. All of them are ways of talking about the link between bells and other kinds of man-made art. For all the talk about bells coming alive in this poem, they are something manmade. We pour them out of metal, and tune them, and play songs on them. What the speaker is really interested in is the way that different sounds can make us feel.
To the pæan of the bells— (line 98)
A "paean" is another word used to describe a musical work of art. It means a song of praise or triumph. In this case, it's a little bit ironic, since the triumph of the ghouls and the misery they create isn't really a good thing. It's all part of the flipping effect in the poem. The speaker is trading the happy ditties of the first two sections for the twisted, triumphant songs of fear and death in the last two sections.