A composer has written a column at the New York Times titled Compose Your Own, He has created a musical composition that he has made into an “open score” inviting readers to electronically mix-and-mash it into their own version.
You can go to a special website to easily create your own musical composition and post it.
I’ve written about how I use these kinds of musical sites as a language-learning opportunity, and you can read about it at The Best Online Sites For Creating Music. I’m not adding this “open score” application, though, because I believe it’s going only be temporary, but I might be wrong. If I am, I’ll add it there later.