Primary Audio vs. Secondary Audio

Primary audio - Audio that is absolutely required by an application. This audio should always play regardless of any other audio being played by other applications running on the system.

Secondary audio - Audio that can enhance an application but is not absolutely required. This audio may be turned off when other applications are already playing audio on the system.

For example, a game may classify its primary audio as sound effects; explosions, bleeps & bloops, short bits of dialog and so on. This is the audio that is absolutely required for gameplay and should always play even if music is being played elsewhere on the system by some other application. The user would expect this primary audio to mix in with the other system audio that is already playing.

This same game would classify secondary audio as its music soundtrack. While the music soundtrack would enhance gameplay, it's not considered mandatory. If the user was previously listening to their own music or podcast the game could choose to mute its own music soundtrack.

By using the secondaryAudioShouldBeSilencedHint property and the AVAudioSessionSilenceSecondaryAudioHintNotification notification, the game can decide when to mute or un-mute its secondary audio soundtrack.

A Voluntary Interruption

You can think of the AVAudioSessionSilenceSecondaryAudioHintNotification as a voluntary interruption. iOS informs your application that another application is playing audio thereby letting your application decide when to mute or un-mute some or all of its audio in response to another application playing audio. This notification provides a AVAudioSessionSilenceSecondaryAudioHintType dictionary key containing a value indicating the type of notification. A begin event called AVAudioSessionSilenceSecondaryAudioHintTypeBegin or an end event called AVAudioSessionSilenceSecondaryAudioHintTypeEnd.

/* keys for AVAudioSessionSilenceSecondaryAudioHintNotification */

/* value is an NSNumber representing an AVAudioSessionSilenceSecondaryAudioHintType */

AVF_EXPORT NSString *const AVAudioSessionSilenceSecondaryAudioHintTypeKey NS_AVAILABLE_IOS(8_0);

/* Used in AVAudioSessionSilenceSecondaryAudioHintNotification to indicate whether optional secondary audio muting should begin or end */

typedef NS_ENUM(NSUInteger, AVAudioSessionSilenceSecondaryAudioHintType)

{

    AVAudioSessionSilenceSecondaryAudioHintTypeBegin = 1,  /* the system is indicating that another application's primary audio has started */

    AVAudioSessionSilenceSecondaryAudioHintTypeEnd = 0,    /* the system is indicating that another application's primary audio has stopped */

} NS_AVAILABLE_IOS(8_0);

When your application is notified of the begin event it is a good time mute your secondary audio. When your application receives the end event it is a good time to resume playback of your secondary audio. The application is in control of what it would like to do when using these hints, there are no behaviors forced upon your application by the audio system.

Reference

What's New In Core Audio WWDC 2014

See AVAudioSession.h which is part of AVFoundation for complete details.