[gdubs]

I've posted this before, but it's relevant to mention AudioBus which is a way to pipe separate music apps together on one iOS device. I've incorporated it into one of my apps and it works really well.

According to Michael, it's creator, it uses Mach Ports to communicate between apps. Apple embraced it and included it with GarageBand.

[alayne]

At this point there is JACK, AudioBus, and soon inter-app audio in iOS7.

[swift]

Apparently AudioBus has already been updated to be interoperable with inter-app audio:

http://createdigitalmusic.com/2013/09/audiobus-for-ios-7-upd...