How they scream out their affright! (line 40)
This is a great image, and a good example of Poe's awesomeness (yeah, we're pretty big fans). Instead of just clanging loudly, the brass alarm bells scream out their fear ("affright"). This use of personification gives a feeling of intensity and danger to this moment. It's as if the fire was so hot and scary that it even brought the bells to life. It's a big change from the happy sleigh rides and mellow weddings we've been hearing about in the first few sections. All of a sudden the monster of fear is on the loose.
What a tale their terror tells (line 52)
First of all, check out the excellent alliteration in this line. There's a tight, nervous sound in all those repeating "t"s. We think it helps to ratchet up the tension, and make us feel the fear that's spreading through the poem. This is Poe's home base, for sure. He made his reputation on tales of terror, and on descriptions of moments just like this one, when fear hangs in the air.
What a horror they outpour (line 55)
Notice the way that the speaker cycles through different words for the fear that the bells feel (and the fear their sound causes). He calls it "affright," "terror," and now "horror." In this case, he's talking about a kind of liquid horror pouring out of the bells. It helps to remember that fire was a really scary and common thing in the nineteenth century, when this poem was written. If you heard a fire alarm bell in the night, you better believe you'd be scared.