When flowing cups pass swiftly round With no allaying Thames, Our careless heads with roses bound, Our hearts with loyal flames; (9-12)
The speaker and his friends' devotion—loyalty—to their king is fiery and powerful, to say the least. The phrase "loyal flames" suggests as much. At the same time, the word "flame" implies that there might be something dangerous about their loyalty. Like, being put in prison, maybe?
When, like committed linnets, I With shriller throat shall sing The sweetness, mercy, majesty, And glories of my King (17-20)
The speaker goes all-out in these lines, referring to the king's "sweetness, mercy, majesty" and his "glories." This is a bold statement of the speaker's loyalty and praise. In addition, the lines themselves are "sweet." Notice the pleasing alliteration (the words that begin with s and m) and the consonance (the l sound) used here.
When I shall voice aloud how good He is, how great should be, Enlargèd winds, that curl the flood, Know no such liberty. (21-24)
Publicly proclaiming one's loyalty ("voice aloud") can be a source of great freedom and power. When the speaker does just that, he is more free and powerful than "enlargèd winds."