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Applications that play audio can continue to play this sound in the background. To indicate that your application is playing background audio, include the UIBackgroundModes key in its Info.plist file. The value for this key is an array containing the sound string. When this key is present, system audio systems automatically prevent the application from pausing when moving to the background. Your application continues to run in the background while it plays audio. However, if this key is missing when the application moves to the background, or if your application stops playing sound in the background, your application pauses.
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