"Mutability" uses a complex metaphor to introduce a complex subject: change. The poem's speaker compares "dissolution"—that is, disintegration—to notes moving up and down a scale, making a strange and "awful" music.
"Awful," in this instance, means "awe-inspiring," not "horrible." But the speaker's choice of this ambiguous word suggests there might be something uncomfortable about the music of change. After all, the kind of change the speaker discusses here isn't new life or rebirth, but "dissolution" and decay. This will be a poem more about endings than beginnings. And endings, as this speaker well knows, are often hard for people to accept.
But accept them they must, the poem implies. Take another look at the repetitions of the first two lines:
From low to high doth dissolution climb,
And sink from high to low, [...]
The chiasmus in these lines suggests that the "music" the speaker is describing here is an eternal circle: the music of "dissolution" makes its way from "low to high," then from "high to low," over and over again. Loss and decay, in other words, are an inescapable constant.
Let's look closer at the language of the metaphor here as well:
- As the reader has already seen, that movement from "low to high" is an image of music—and an image of the change and "dissolution" that's necessary to music. Without high and low notes, there's no tune, just a drone. And the previous note has to dissolve away in order for the next note to take its place.
- But the same "high and low" metaphor is also a reminder that "dissolution" affects everything and everyone, from the greatest kingdom to the lowliest bug.
In only three lines, then, the speaker has introduced both his theme and the complex emotions people might have around it. "Dissolution," this poem will suggest, can feel terrifying and painful. But it's also an eternal part of the universe—and seen in a certain light, it's even as beautiful as music. The "awful notes" of change create, not shrieks of grief, but a "concord" (that is, a harmony) that "shall not fail": an eternal song.