Adding a new sound to the mixer looks complicated but it's actually quite easy.
Based on the sample project, MixerHost, you will need to make some changes.
0) in MixerHostAudio.h
i) change file number accordingly
1) in MixerHostAudio.m
i) add a new NSUrl to the bundle resource
ii) modify enableMixerInput, such that it knows what to do when an extra input is on
iii) change the busCount accordingly
2) in MixerHostViewController (.h, .m)
i) Add switches and faders accordingly.