AVAudioPlayer - returns true, but there is no sound

Question

I am trying to play sound when user enters background:

let dispatchQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    dispatch_async(dispatchQueue, {[weak self] in
        var audioSessionError: NSError?

        let audioSession = AVAudioSession.sharedInstance()
        NSNotificationCenter.defaultCenter().addObserver(self!, selector: "handleInterruption:", name: AVAudioSessionInterruptionNotification, object: nil)
        audioSession.setActive(true, error: nil)

        if (audioSession.setCategory(AVAudioSessionCategoryPlayback, error: &audioSessionError)) {
            println("Successfully set the audio session")
        } else {
            println("Could not set the audio session")
        }

        let filePath = NSBundle.mainBundle().pathForResource("sound", ofType:"mp3")
        let fileData = NSData(contentsOfFile: filePath!, options: .DataReadingMappedIfSafe, error: nil)
        var error:NSError?

        self!.audioPlayer = AVAudioPlayer(data: fileData, error: &error)

        self!.audioPlayer?.numberOfLoops = -1

        self!.audioPlayer?.delegate = self

        if (self?.audioPlayer?.prepareToPlay() != false) {
            println("Successfully prepared for playing")
        } else {
            println("Failed to prepare for playing")
        }
    })
    if (self.audioPlayer?.play() != false) {
        println("Successfully started playing")

    } else {
        println("Failed to Play")
    }

Both player.prepareToPlay() and player.play(), they return true. But actually there is no sound. If I run this anywhere else, it works. Help me please. I have added background audio mode.