OK, so the title isn’t really a quote from the poem, but we have to include it because the word "mutability" only appears in the title. Mutability is an abstract word that alludes to a tradition of dealing with time and death in English poetry. Wordsworth is like, "OK, guys and gals, it’s my turn to take on the whole transience thing, just letting you know."
From low to high doth dissolution climb (line 1)
"Dissolution" is another word for transience in the poem. Think "dissolve," like a tablet of Alka Seltzer dissolving in water. Time creates change by wearing away at old things.
A musical but melancholy chime (line 4)
The comparison between dissolution and music is appropriate because change is necessary for music to exist. Imagine a song with only one note or chord continuing to play endlessly. Would you want to listen to that song? Or, as the American poet Wallace Stevens later put it, "Death is the mother of beauty."