Since the introduction of OS X 10.11 (El Capitan) it is no longer possible for AudioSwitcher and many other applications to interrogate AirPlay devices and use them as you would other audio devices on your Mac. This problem is even present in Apple’s own applications such as Audio Midi setup.
Using kAudioHardwarePropertyDevices no longer returns a device ID for AirPlay. Although you can get a handle to the device ID by using kAudioHardwarePropertyTranslateUIDToDevice you can no longer enumerate that devices data sources, i.e, the AirPlay devices attached to the system.
At this point it’s not clear whether this is a bug or a deliberate removal of functionality by Apple. Developers around the world (including myself) have raised this issue as a bug with Apple but as of early November there is no clear picture as to when or even if this will be fixed.
If you open System Preferences then the Sound pref panel it seems that AudioSwitcher and other apps can then enumerate AirPlay devices. You may need to restart your application to see the AirPlay devices.