"As a dare-gale skylark scanted in a dull cage
Man's mounting spirit in his bone-house, mean house, dwells--
That bird beyond the remembering his free fells;
This in drudgery, day-labouring-out life's age.
"Though aloft on turf or perch or poor low stage,
Both sing sometimes the sweetest, sweetest spells,
Yet both droop deadly sometimes in their cells
Or wring their barriers in bursts of fear or rage."
(lines 1-8 of "The Caged Skylark" by Gerard Manley Hopkins)
Gerard Manley Hopkins uses sound in his poems to make meaning. Alliteration, repetition, and the occasional cacophony are key. Almost always, the sound of the words directly and overtly mirror what they mean.
In line six, the "s" sound recurs and by "sweetest, sweetest spells," this causes the poem to sound soft and sweet. Yet he uses alliteration for the exact opposite in the second line where the recurring "m" and "n" sounds drone and almost buzz, investing the line with a harsh, defiant tone. The repetition of "bone-house, mean house" makes it all the more insistent. If he had used, say, the "s" effect in that line instead, it would alter the tone and meaning of the whole poem by making it sound more soft and resigned.
My favorite line is "This in drudgery, day-labouring-out life's age." It's really hard to say, and the hyphenated "day-labouring-out" slows the line down to a crawl. All of this combines to really illuminate the kind of slow, futile, dragging imprisonment of the "dare-gale skylark" and the human soul.