AVAudioPlayer播放音頻文件時沒聲音

AVAudioPlayer播放一個mp3文件時，居然沒有聲音。mp3文件是放在工程里面的，路徑沒有錯誤但就是死活沒有聲音。

func playSound() {
        let notifyUrl = NSBundle.mainBundle().pathForResource("notify", ofType: "mp3")
        //let notifyUrl = NSBundle.mainBundle().resourcePath?.stringByAppendingPathComponent("notify.mp3")
        if let mp3 = notifyUrl {
            let url = NSURL(fileURLWithPath: mp3)
            
            //            var filemanager = NSFileManager()
            //            let result = filemanager.fileExistsAtPath(mp3)
            //            println("result=\(result)")
            
            AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback, error: nil)
            
            var error:NSError?
            var avAudioPlayer = AVAudioPlayer(contentsOfURL: url, error: &error) 
            if error != nil {
                println(error)
            } else {
                avAudioPlayer.volume = 1.0
                avAudioPlayer.prepareToPlay()
                avAudioPlayer.play()
            }
        }
    }

 解決方法是把avAudioPlayer定義為全局變量的時候就有聲音了，是不是很神奇。應該是被內存回收了所以沒有聲音，記得要定義為全局變量

var avAudioPlayer:AVAudioPlayer?
func playSound() {
        let notifyUrl = NSBundle.mainBundle().pathForResource("notify", ofType: "mp3")
        //let notifyUrl = NSBundle.mainBundle().resourcePath?.stringByAppendingPathComponent("notify.mp3")
        if let mp3 = notifyUrl {
            let url = NSURL(fileURLWithPath: mp3)
            
            //            var filemanager = NSFileManager()
            //            let result = filemanager.fileExistsAtPath(mp3)
            //            println("result=\(result)")
            
            AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback, error: nil)
            
            var error:NSError?
            //var avAudioPlayer = AVAudioPlayer(contentsOfURL: url, error: &error)
            avAudioPlayer = AVAudioPlayer(contentsOfURL: url, error: &error)
            if error != nil {
                println(error)
            } else {
                avAudioPlayer?.volume = 1.0
                avAudioPlayer?.prepareToPlay()
                avAudioPlayer?.play()
            }
        }
    }