The typical Slashdot thread, where a bunch of semi-related facts get thrown in a blender and nothing good comes out.
Indeed, hasn't music *always* been open source?
In the sense of one composer re-using parts of someone else's work, yes until recently. Imitation was flattery, and it has always been clever to quote something. Open source in the sense of "take this and do stuff with it"? No.
Composers have not been publishing their sheet music for everyone to see.
Composers have been publishing their music, but not for everyone to see. For everyone to buy, yes, but not see freely.
Actually historically many (most) composers did publish sheet music - it was one of the few ways they could make money from their art, conducting and taking commissions being the others.
It sounds like you are making a correction, but you aren't, unless you misread the "for everyone to see" part.
And anyway why would they need to publish their sheet music for others to build upon their work? Sheet music is not the source code, it's the note-cards you take to the podium.
Apparently you can memorize every note by hearing it once or twice - please go to the nearest university psychology department. They will want to scan your brain and see what makes you extremely rare, like one in hundreds of million rare. Or maybe you can remember the key and general shape of a complete work by hearing it once or twice - still on the order of one in 10,000 at the most generous.
any decent composer can listen to a piece only once or twice before creating their own composition clearly inspired by it
Oh, that's all you meant? That's not open source, that's art. You're talking about downloading a tarball of Quake 3 and making a completely new game, art and all. A mostly new work with foundations in the original. "Variations on a theme of" is one of the most popular ways to start a title for work like this, and no music is needed.
If you just need to patch two measures of Beethoven because there's just no way he was *that* dissonant, and you're pretty sure the original manuscript was just mis-read, good luck doing that without music. That is what open source implies.