How we cite our quotes: Book Number, Line Number
Quote #1
"If once they hear that voice, (their liveliest pledge
Of hope in fears and dangers […] they will soon resume
New courage and revive" (1.274-5; 278-9)
The power of Satan's voice is an important theme throughout Paradise Lost. Here, the emphasis is on the actual sound of Satan's voice and how it renovates the fallen angels' despair. At other moments in the poem his voice is just as effective, though it achieves different results; he uses it to trick Eve, for example, in Book 9, whereas in the early books his speeches seduce us (as readers) into admiring him.
Quote #2
"His thoughts were low,
To vice industrious but to nobler deeds
Timorous and slothful. Yet he pleased the ear
And with persuasive accent thus began" (2.115-8)
Much like Satan, Belial (described here) is a bad dude; he's just as dangerous too because he "pleases the ear" with "persuasive accent[s]." Milton often points out the way in which what is "pleasing" can cause us to ignore someone's love for "vice." The voice, not just Satan's but God's as well, is a very powerful force in Paradise Lost.
Quote #3
"what surmounts the reach
Of human sense, I shall delineate so,
By likening spiritual to corporal forms,
As may express them best, though what if Earth
Be but the shadow of Heav'n, and things therein
Each to other like, more than on earth is thought?" (5.571-6)
Raphael uses similitude to give some idea of what Heaven is like, knowing full well that this is at best an imperfect approximation. Raphael seems a lot like John Milton, who must have faced the same exact problem of trying to explain "spiritual" things in earthly or "corporal" terms.