How the danger ebbs and flows; (line 60)
Fire was a big threat in Poe's day, and the chance that you would die in a fire, or lose your whole house, was much greater than it is today. Even today, fire is a scary and unpredictable thing. In this moment in the poem, the power and danger of fire is stalking through the town, and we know that death is marching along with it. At this point death is lurking just behind the fire. It's an invisible threat in the poem, but we can feel its presence.
What a world of solemn thought their monody compels! (line 72)
The word "monody" has heavy overtones of death. It can mean a poem in which the speaker laments someone's death, or a mournful song. In this case, the song of the bells is forcing the people listening to it to think "solemn thoughts." In our experience, that's just how thoughts of death creep into the mind. The speaker weaves the thread of death into the poem with the lightest possible touch. He picks just the right words to hint that death is slipping out of the bells like a ghost.
At the melancholy meaning of their tone! (line 75)
Behind the tone of the bells, the wordless sound they make, there's some kind of "melancholy meaning." We can be pretty sure that these bells are ringing for some sad occasion. Given the little hints the speaker drops, we think it's pretty safe to assume that the iron bells are ringing for a funeral. Maybe the occasion isn't as important as the pure feeling of the sound, the heavy misery that the bells broadcast. It's a totally different feeling from the frantic fear of the fire alarm bells in section 3.