Lines 71-74 Summary

Get out the microscope, because we’re going through this poem line-by-line.

The Bleat, the Bark, Bellow & Roar
Are Waves that Beat on Heaven's Shore.

Now, we're finally back to animals—or, at least, to animal noises.
This is another weird, transcendental, left-field couplet. It's sort of related to the four lines that came before it, though: Blake is saying that the emotions and powers at work on Earth reverberate in Eternity or heaven (or something like that). What happens in the world has an effect in the Beyond.
Or, he could be saying that the passions and terrors of this world—which are symbolized by the animal noises—are like a tumultuous ocean. But heaven exists beyond that—it's the safety of the shore.
Or, he could really mean both of those things at once.
There's some alliteration in this couplet, too ("The Bleat, the Bark, Bellow"). Also, we get a sort of internal rhyme with "bleat" and "beat." Check out "Sound Check" for more.

The Babe that weeps the Rod beneath
Writes Revenge in realms of death.

This couplet is pretty dark, since it deals with child abuse. The baby who gets beat with a rod (think stick or switch) causes a cosmic reaction to happen: the person who hit the baby (probably a parent) will get punished in death, experiencing the same pain that they'd caused in life. The baby will get revenge, payback.
On the other hand, you can read the last line as meaning that the baby will die and "write revenge in realms of death" in the afterlife, preparing punishment for the abuser. But the first interpretation probably makes more sense—it's to the point.
This is a "world in a grain of sand" moment: a horrible but common event—child abuse—causes a reaction in eternity, or the beyond.
Again, this couplet includes another rhyme ("beneath" and "death") that isn't a rhyme today.