Kazoo
This fully-3d-printable kazoo takes skill to master and frankly, would work much better, if instead of printing the "membrane" component, you were to use wax paper. However, the rules of the make it loud challenge prohibit non-3d-printed components and therefore we have to make do with the thin circle of filament. I'm still working on making it better/more flexible.
Remember to hum into it rather than just blowing!