Now we just need to associate payment/attribution related metadata with those identifiers

Content addressable doesn't really work here... different apps may have the same recordings encoded in different formats and bitrates, but they are still the same recording. Unless you meant "content addressable" in the sense of a uniquely assigned identifier like I was already talking about, and not a computed identifier from the raw bytes of the file like a hash.

This sounds like an acoustic fingerprint, such as AcoustID[0]. I think AcoustIDs and XSPF[1] would be a good combination for shared playlists. It's a shame that development stopped on the Tomahawk music player[2], it would have been an ideal platform for shared playlists like this.

[0] https://musicbrainz.org/doc/AcoustID

[1] https://en.wikipedia.org/wiki/XML_Shareable_Playlist_Format

[2] https://github.com/tomahawk-player/tomahawk