"Notes of woe" recurs in Blake's immensely complex The Four Zoas, in Vala's attempt to mollify Tharmas's fury:
Tharmas replied Vala thy Sins have lost us heaven & bliss
Thou art our Curse and till I can bring love into the light
I never will depart from my great wrathSo Tharmas waild wrathful then rode upon the Stormy Deep
Cursing the Voice that mockd him with false hope in furious mood
Then She returns swift as a blight upon the infant bud
Howling in all the notes of woe to stay his furious rage
(94:24-30)
In "THE Chimney Sweeper" the "notes of woe" are also an instrument of pacification, though they're sung on behalf of the powers that have dressed the child in "clothes of death." If you've looked at the annotations to "The Chimney Sweeper" of Songs of Innocence (linked from the index frame on the left), you've seen that the "clothes of death" are literally so. This is a more explicitly angry poem than the earlier one, with an undercurrent of bitterness that would have been wholly foreign in the first Sweeper poem. Anger is an emotion of experience, not of innocence, and if we accept Blake's itinerary of consciousness as a journey from innocence, through experience, terminating in a higher, sublime innocence, we can see why: the child-sweeper of Experience is forcibly rapt up out of the prelapsarian state of happiness ("Because I was happy upon the heath, / And smil'd among the winters snow") and bartered into a world of frozen cruelty.
A great deal hangs on that word "Because": the child locates the cause of his predicament in his own happiness. He's left with a residual awareness of his innocence, and indeed, with the capacity to access the world of innocence in a degree. But, as we'll see in the last stanza, this isn't any felix culpa--Fortunate Fall--but rather a grim understanding of his station in the world of experience.