It's not enough to only add the audio
background mode. You must also set a playback audio session category, as described in Apple QA1668:
A: You must declare your app plays audible content while in the background, and assign an appropriate category to your audio session.
#import <AVFoundation/AVFoundation.h>
#import <AudioToolbox/AudioToolbox.h>
AVAudioSession *audioSession = [AVAudioSession sharedInstance];
NSError *setCategoryError = nil;
BOOL success = [audioSession setCategory:AVAudioSessionCategoryPlayback error:&setCategoryError];
if (!success) { /* handle the error condition */ }
NSError *activationError = nil;
success = [audioSession setActive:YES error:&activationError];
if (!success) { /* handle the error condition */ }