Adding a sound in the Crystal Ball app

So what's happening is this line figures out where the sound file you want to play is located in the iPhone's filesystem:

NSString *soundPath = [[NSBundle mainBundle] pathForResource:@"crystal_ball" ofType:@"mp3"];

Then, this line converts the NSString named soundPath to a usable URL to where the file is located in the iPhone filesystem:

NSURL *soundURL = [NSURL fileURLWithPath:soundPath];

This must be done because the call on the next line only accepts URLs, and not strings. Finally, this C function creates a SystemSoundID for the file that you want to play, so you can play it later with AudioServicesPlaySystemSound(soundEffect);:

AudioServicesCreateSystemSoundID(CFBridgingRetain(soundURL), &soundEffect);

Admittedly, that is a little complex, especially the last line. To break down the last line, AudioServicesCreateSystemSoundID(); is called to create a SystemSoundID for the file you passed in. This C function takes 2 parameters: a URL, and the memory address of the variable that you want the sound assigned to. Since it is a purely C function, the URL must be converted into a format that can be used by C, which NSURL is not. Therefore, CFBridgingRetain() is called on the NSURL named soundURL to convert it to a format usable by C. What that function returns is then passed into AudioServicesCreateSystemSoundID(); as its first parameter. The second parameter that is passed into it is &soundEffect, which is the memory address (as denoted by the "&") of the variable named soundEffect, which was declared at the top of the @implementation, like this:

@implementation THViewController{
    SystemSoundID soundEffect;
}

The sound ID (which can be thought of as the sound itself) is assigned to that variable, so it can later be played like this:

AudioServicesPlaySystemSound(soundEffect);