With a crystalline delight; (line 8)
We think the phrase "crystalline delight" sounds like pure, concentrated happiness. It just naturally makes us want to smile. The sounds are crisp and pleasant, and the image it calls up in our minds is bright and clean. This is the essence of the mood at the opening of this poem: clear, cheerful, and totally unspoiled by fear or sadness or death. At this point in the poem you might be wondering, "Did Edgar Allan Poe really write this poem?"
What a world of happiness their harmony foretells! (line 17)
At this moment, the bells are forecasting a kind of endless happiness, spreading all over the world. That makes sense, since they are ringing for a marriage. That's normally a moment of excitement and possibility. Ideally, it's also a moment of "harmony," where the love of two people chimes together like bells ringing in tune.
How they ring out their delight! (line 19)
This is another moment of pure, unspoiled joy. The bells come alive with happiness. They are filled with "delight," which we usually think of as a human feeling. The point is that the whole poem and the whole world it describes are just drenched in happiness at this point. Poe is setting us up for a pretty big fall, but we don't know that yet. We're only seeing one side of the coin – the joy that can't last forever, but that feels like it's going to.