Last year we created software synthesizer app for iPad called Orbit.io, which enables the user to easily create loop-based musical patterns and melodies with an unique interface. It was now put on Design made in Germany and you can check it out there. We developed this app together with students of the communication-design department in our university, HTW Berlin. The graphical user interface was done using cocos2d and the audio backend was driven by a self-developed library based on top of Stanford University’s synthesis toolkit. See the nice video made by communication-design students Dennis Michaelis and Martin Guder:
The audio backend was mostly written by me and I plan to release a cleaned up and portable open-source variant of it later on. The task was quite challenging since audio synthesis back in the days of the limited resources of the iPad I was really a matter of CPU and memory optimization. See also my paper on this topic.
Unfortunately, the app never left the beta-status and is not available for the public but maybe someday this will change.