This short little poem "Upon Julia's Voice" by Robert Herrick really has only one main subject, Julia's voice. The poem also makes mention of "the damned."
Herrick is enchanted by her voice which he describes using alliteration "so smooth, so sweet, so silv'ry." The repeated soft consonant sounds mimic her light and ethereal voice which he likens to "melting melodious word."
Herrick uses the "damned" as a contrast to Julia's heavenly voice, telling the reader that if these fallen souls could hear, then they would stop and listen. This approach really is quite effective, because he chooses the lowest of the low as her possible audience to suggest that even they, the hopeless and the lost, would be charmed by her voice.