Audio playing not working when ios device is locked - avaudioplayer

Question

I am doing the following things to make audio play when an iOS device is locked, but it doesn't work.
UIBackgroundModes key (with the value audio) in Info.plist file.
set up audio session
code:
- (void)viewDidLoad
{
[super viewDidLoad];
[self createAudioSession];
NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:#"jgj" ofType:#"mp3"];
NSURL *url = [NSURL fileURLWithPath:soundFilePath];
NSLog(#"%#",[url path]);
NSError* error;
self.audioPlayer=[[AVAudioPlayer alloc] initWithContentsOfURL:url error:&error];
if (self.audioPlayer == nil)
NSLog(#"%#",[error description]);
else
{
self.audioPlayer.numberOfLoops=-1;
[self.audioPlayer play];
}
}
-(void)createAudioSession
{
AVAudioSession *audioSession = [AVAudioSession sharedInstance];
// Use this code to allow the app sound to continue to play when the screen is locked.
NSError* error=nil;
BOOL ok=[audioSession setCategory: AVAudioSessionCategoryPlayback error: &error];
if(!ok)
NSLog(#"%#",[error description]);
else
NSLog(#"hello");
ok=[audioSession setActive:YES error:&error];
if(!ok)
NSLog(#"%#",[error description]);
else
NSLog(#"world");
}