Actually I don't know, how this is done with Cocos Creator, but maybe it is helpful to know how it's done in cocos2d-x.
You have to set the AudioSessionCategory, if I remember correctly.
Add the following line to
RootViewController.mm right under
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryAmbient error:nil];
Of course you also need to add
to the top.