All the examples of the Learning Core Audio book rewritten with Swift 2.0
If you use different devices for input and output the output can be silence. In such case try to increase or decrease size of the ring buffer.
The MIDIWifiSource is crashing with iOS 9 + OS X 10.10 (I've no possibility to test it with another configurations). The problem is in creating of connection (line #50):
let connection = MIDINetworkConnection(host: host)
I didn't found how to fix it yet.