An antecedent of a pronoun is a word that refers to the same thing as this pronoun.
here the pronoun is "theirs"
<em>Rivers stopped flowing along the bank's so that they could hear Orpheus, whose music was harmonious than </em><em>theirs.</em> (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)
