Line 3: "His breast was bare" combines alliteration with visual imagery to evoke feelings of sympathy in the reader.
Line 9: "Beneath the palm-trees on the plain" is geographic alliteration to help identify the slave's native land.
Line 16: "They held him by the hand!--" is more alliterative body imagery, used with an exclamation point, to evoke more emotion (pathos) from the reader.
Lines 23-24: "scabbard of steel / Smiting his stallion's flank." combines alliteration with weapon imagery to show the destructive force of the military. (Although one may call repetition of -s sound as sibilance instead of alliteration)