The poem begins with a strange, bold claim: the mouths of poor people are "a considerable instrument." It treats speech, or perhaps hunger, as something like music, a thing that can be played and heard. Then the comparisons shift. The mouths are like "absence" and like "doors in a wall," and the wall is "so high and melancholy" that it troubles the heart.
The speaker doesn't explain these images, so the reader has to feel their way through. A door in a high wall suggests an entrance that is out of reach, or a way through that stays closed. Absence suggests what is missing from poor people's lives, and perhaps what the rest of us fail to hear. Judging by the title, "Soulful," the poem hears something deep and mournful in these voices.
The mood is quiet, sad, and uneasy. The speaker seems moved and a little shaken, as if the sight of those mouths, and the barrier around them, is hard to ignore.