An antecedent of a pronoun is a word that refers to the same thing as this pronoun.
here the pronoun is "theirs"
Rivers stopped flowing along the bank's so that they could hear Orpheus, whose music was harmonious than theirs.
(all the words in Boldface refer to the same thing).
What does the "theirs" refer to? it refers to their music, and the they is the Rivers (it's a personification of rivers)
The antecedent is the river!
