After iOS 7 presentation Apple posted some details on Inter-App Audio.
Now your apps can make beautiful music together. With Inter-App Audio, apps can register their audio streams to share with other apps. For example, a series of apps could publish audio streams of instrument tracks while another uses the combination of these streams to compose a song. Inter-App Audio also provides for MIDI control of audio rendering, remotely launching other registered Inter-App Audio apps and more.
UPDATE: Here’s good explanation on how it could work: http://discchord.com/blog/2013/6/11/inter-app-audio-again.html