soundIdentifierForFocusUpdateInContext:

The identifier of the sound to be played. Return nil if you want to let the parent focus environment determine which sound to play.

Use this method to return a custom sound when the focus environment object gains focus. Return UIFocusSoundIdentifierDefault to play the default system sound, or return UIFocusSoundIdentifierNone to avoid playing a sound altogether. If you previously registered custom sounds using the registerURL:forSoundIdentifier: method of UIFocusSystem, you may also return an identifier for a sound that you registered.

If you do not implement this method, the system assumes a nil return value. If no ancestor environment defines a custom sound, the system plays the UIFocusSoundIdentifierDefault sound.