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.