Chrome Music Lab has a little bit of art and music combined. It's a safe space for students to play around with harmony, rhythm, chords, etc. It's a chrome experiment, so right now there's not an easy way to save work. But you might be able to record your screen and mic with some promising results!

Flat allows you to search for, create, and publish sheet music! It's free, and it's really easy to use. I even composed a short piece called "Wiggle" that has brass and percussion.