Smule, the company behind the popular Ocarina application that turns your iPhone into a musical instrument, has released a new app called Leaf Trombone World Stage. Ge Wang, the Stanford professor who co-founded the company, likes to talk about his vision for a Smulean future, i.e. one in which music-making is more accessible and social — and Leaf Trombone definitely takes the iPhone further in that direction.

At the center of the Leaf Trombone app is a new musical instrument. Just as the Ocarina app was based loosely on the real-world Ocarina instrument, the Leaf Trombone sounds kind of like an actual trombone. But Smule has gotten a little more creative with its interface this time — you move a slider up-and-down a leaf to make the desired sound.

The company says it was inspired by “traditional Chinese leaf instruments.” You can either blow into the iPhone microphone to play or set the app to “touch” mode, where no blowing is required (making the app both easier to learn and also playable on the iPod touch). After watching a few videos and testing the app myself, I can say that the instrument certainly seems versatile, if less aesthetically appealing than its Ocarina cousin.

More interesting are the features that Smule has built around the instrument — namely, the “World Stage” part. With Ocarina, you can share what you are playing with other users, but the Menlo Park, Calif. company has formalized that process with Leaf Trombone, allowing other users to rate performances on a 1 to 10 scale. These ratings contribute to a player’s ranking in the Leaf Trombone community.  The app’s interface also makes it easy to play other compositions, with the notes represented by smaller leaves flying at your instrument, showing you where to move the slider. There’s even a way to compose songs for the Leaf Trombone in your web browser.

Smule first demonstrated the Leaf Trombone at Apple’s iPhone 3.0 event last month. It seemed like a bit of an odd fit at the time, because the app didn’t seem to do much that was interesting with the new developer tools — it just allowed in-person duets using the Connection application programming interface. Now that Leaf Trombone is out prior to the actual release of the iPhone 3.0 operating system, it’s even clearer that the 3.0 features aren’t a key part of the app. I guess Apple just brought Smule on stage because it’s fun and cool.

The company has raised a total of $5.7 million in venture funding.