It’s not just that copies of a song aren’t rivalrous. It’s that the more copies there are, the more the song is worth.
That’s because one of the functions of music is to be cultural currency. Michael Jackon’s “Thriller” is a good conversational topic. Beethoven’s Ninth Symphony is a landmark that we’d use to describe similar, but less known, works. Thriller and Beethoven’s Ninth wouldn’t be useful in those ways if they weren’t so well known.
Shared references are the power source for mashups like Girl Talk. Because everybody in the crowd recognizes the source samples, the audience can understand they music. The better known the song is, the more people get the reference. And the more copies, the better known.