Using OpenID, the composer could assert authorship rights. It doesn’t stop anyone from using (e.g. via the HTML5 audio src attribute) the music in another page or feed.

The question is how we could get there; it seems a bit far off! But following this line of thinking certainly leads to interesting business plans…

Yes, MusicBrainz is exactly that. It’s a metadatabase for song/track/artist information (and more) edited by the community in a wiki-like way (an they’re willing to do so since several years already). It provides unique identifiers for all the bits and pieces of music to be re-used in other catalogues. The audio signatures are actually just a small part of what it’s all about.