Play sound when app is in background or screen is locked - objective-c

Question

I have a timer that plays a sound when time reaches 0, but now I can't understand how to fire the event in order to play the sound if the app is in background mode or the screen is locked, What I found searching on the web was to set "UIBackgroundModes" in my .plist file, and add "audio" as array member.
UIBackgroundModes Array
Item0 String audio
Then add in the app delegate the following code:
NSError *setCategoryErr = nil;
NSError *activationErr = nil;
[[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryPlayback error:&setCategoryErr];
[[AVAudioSession sharedInstance] setActive:YES error:&activationErr];
But the sound doesn't get played.. So I was wondering if it's possible to do something like this or I should change method
EDIT:
I imported:
#import <AudioToolbox/AudioToolbox.h>
#import <AVFoundation/AVFoundation.h>
And this is the code I use to create the sound:
AVAudioPlayer *player;
NSString *soundFilePath = [NSString stringWithFormat:#"%#/clock.mp3",
[[NSBundle mainBundle] resourcePath]];
NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
player = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL
error:nil];
player.numberOfLoops = -1;
dispatch_async(dispatch_get_main_queue(), ^{
[player prepareToPlay];
});
Then to play it:
[player play];

Answer 1

I have just a snippet of Swift code but it should be easy to convert it to Obj-C
Try to use AVPlayerItem init with URL and then set AVPlayer init with PlayerItem
let assetUrl = self.nowPlayingItem!.associatedItem!.valueForProperty(MPMediaItemPropertyAssetURL) as! NSURL
let playerItem = AVPlayerItem(URL: assetUrl)
if let player = self.player {
player.replaceCurrentItemWithPlayerItem(playerItem)
} else {
self.player = AVPlayer(playerItem: playerItem)
}
self.player!.play()
and for this line of code:
NSString *soundFilePath = [NSString stringWithFormat:#"%#/clock.mp3",
[[NSBundle mainBundle] resourcePath]];
Have you tried:
[[NSBundle mainBundle] URLForResource:#"song" withExtension:#"mp3"];

Answer 2

// Create a new player item
NSURL *assetUrl = [nowPlayingItem valueForProperty:MPMediaItemPropertyAssetURL];
AVPlayerItem *playerItem = [AVPlayerItem playerItemWithURL:assetUrl];
// Either create a player or replace it
if (self.player) {
[self.player replaceCurrentItemWithPlayerItem:playerItem];
} else {
self.player = [AVPlayer playerWithPlayerItem:playerItem];
}
nowPlayingItem is here a MPMediaItem but just replace NSURL *assetUrl = [nowPlayingItem valueForProperty:MPMediaItemPropertyAssetURL]; by NSURL *assetUrl =[[NSBundle mainBundle] URLForResource:#"song" withExtension:#"mp3"];