There's no CoreAudio SDK, any reference to that is somewhat a remnant of the past (pre iOS) where there was actually an SDK containing tools, samples, helper classes and so on. These bits are all still available separately which is far more manageable and allows for faster updates if say there's a bug in one of the helper classes or tools etc.
The Audio Tools which include AULab and HALLab are avalable here: https://developer.apple.com/downloads/?name=Audio
Note: Xcode will take you to these resources from the menu Xcode->Open Developer Tool->More Developer Tools...
The Core Audio Utility Classes containing the Public Utility sources (PublicUtility folder) as well as base classes required for codec and audio unit development are available here: https://developer.apple.com/library/ios/samplecode/CoreAudioUtilityClasses/Introduction/Intro.html
These utility classes are used by various Apple Core Audio sample project and extend or wrap Core Audio API's.
Note: A few of these helper classes may have been superceded by framework provided objects in OS X Yosemite and iOS 8, for example the AVFoundation Audio Class AVAudioFormat supporting the introduction of "common formats" as opposed to the older "canonical" formats, developers using CAStreamBasicDescription may want to move their code over if appropriate.
Apple sample code can be found by searching the iOS or OS X Developer Reference Libraries. Not only that, but a few quick google keystrokes will find a wealth of code supplied by other developers who have been nice enough to share their Core Audio experiences with the community.
So, you don't need a special SDK to write Core Audio code but you will need to be very self motivated and depending on what APIs you're planning to use and what you're attempting to do there may or may not be many Apple specific resources to just read. Higher level APIs tend to be the best documented while Audio Units, CoreMIDI, Audio Codecs, Drivers and so on (basically the APIs that have been around for 15 years or so) don't really have "Programming Guides" associated with them or if they do are probably out of date.
Here's a place to start with Apple Docs: https://developer.apple.com/library/mac/documentation/MusicAudio/Conceptual/CoreAudioOverview/Introduction/Introduction.…
Finally, I have to mention Chris (Core Audio is Hard) Adamson and Kevin (No seriously dude, Core Audio is Hard!) Avila who authored a very nice introduction to the technology called "Learning Core Audio: A Hands-On Guide to Audio Programming for Mac and iOS" http://www.amazon.com/Learning-Core-Audio-Hands-On-Programming/dp/0321636848
Maybe if we encourage them, we'll get a second expanded edition of this book eh?
As for your question - hog mode is about exclusive access to an audio device (search headers for kAudioDevicePropertyHogMode), but if you don't know why you would need it you probably don't. As one of the Core Audio engineers wrote answering a similar question "...why do you need hog mode for your app? Hog mode is a really unfriendly thing to assert on the system. As such, it should only be used in circumstances where it is essential like sending non-mixable data (such as AC-3) to the hardware. The vast majority of apps are best off pretending like hog mode doesn't exist.”
