Smule, which makes interactive music applications for the iPhone, has raised $3.9 million in a second round of funding.

The Menlo Park, Calif. company made a splash last year with its Ocarina app, which turns your iPhone into a musical instrument and allows you to share and listen to Ocarina music from around the world. Ocarina was one of the best-selling paid apps in Apple’s App Store and showed up on virtually every year-end top 10 list (although it only managed to snag an honorable mention on VentureBeat writer MG Siegler’s list).

Before releasing Ocarina, Smule created the popular Sonic Lighter app, and afterwards it released a holiday app called Zephyr, which lets users create music and messages by touching the iPhone screen.

The company, whose full name is SonicMule was co-founded by Jeffrey Smith and Ge Wang, who’s also a music professor at Stanford University. When I interviewed Wang this December, he didn’t offer any specifics about the company’s future plans, saying only that Smule wants to continue helping people express themselves through music.

The new round was led by Granite Ventures, with participation from existing investors Bessemer Venture Partners, Maples Investments, and Smith (the company’s co-founder). Smule has raised $5.7 million total.

I’ll leave you with this video of someone performing an Ocarina duet (with himself) of Katy Perry’s “Hot N Cold.”