see the UDN .Setting bOtherAudioPlayingNow to always false can workaround the problem, but it's better to have a more robust solution.
1、turn off the silent mode of your iphone
2、4 tap call out console input, type in 'stat unit'
3、start the game, and send some messages or emails to that iphone in order to create notifications, watch the frame time
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-111011 in the post.