ambient
The category for an app in which sound playback is nonprimary — that is, your app also works with the sound turned off.
Declaration
static let ambient: AVAudioSession.CategoryDiscussion
This category is also appropriate for “play-along” apps, such as a virtual piano that a user plays while the Music app is playing. When you use this category, audio from other apps mixes with your audio. Screen locking and the Silent switch (on iPhone, the Ring/Silent switch) silence your audio.