Lines 17-20 Summary

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

The Game Cock clipp'd and arm'd for fight
Does the Rising Sun affright.

Yeah, it's another bird—this time, a rooster. And the poem continues on the same "world in a grain of sand" theme: an apparently little thing has a bigger impact or resonance or significance. Whereas the wounded skylark made an angel fall mute, the rooster, armed for cockfighting, scares the sun itself.
In this case, though, the rooster brings together a bunch of things we've already seen before. Freedom being crushed? Check. Cruelty? Check. Suffering? Check. Violence? Check. The human capacity for evil (since humans organize cockfights)? Check!
The victimized ready-to-fight rooster puts pain and human wickedness front and center. No wonder he's freaking the sun out—especially since the sun's rising, waking up from a long night and expecting to look on a fine, fresh world without all these horrors infesting it.

Every Wolf's & Lion's howl
Raises from Hell a Human Soul.

We're still in Woodland Critter Fun Time Hour. But this one's a little different from the couplets that came before it. It's not an example of someone doing something cruel to an animal (unless some human is doing something to the wolves and lions, making them howl), rather it's an example of animals doing some pretty typical animal stuff—namely, howling.
But this howling has a cosmic effect: it's raising human souls up from hell. So, that puts it more in line with all these other animal couplets—a seemingly little thing having a bigger implication. Blake didn't believe in eternal hell, so he uses "hell" here to mean a state of mind, a state of suffering. The outburst of energy, or of life-force, or of imagination that these howls symbolize can help yank people's souls out of this state (or, at least, that's one interpretation).
"Howl" and "soul" are likely another example of an old-timey rhyming pair. (Also, there's a similarity between "howl" and "hell" that adds something, sound-wise, to the poem. Check out "Sound Check" for the details.)