AudioToolBox运行良好，但现在使音频声音非常安静

Question

我刚写了这段代码。按一个按钮听到声音。我导入了AudioToolBox。我打开并使用我的应用程序的前10次工作正常，但现在由于某种原因，听起来我的音频非常安静，即使我的手机上的音量一直很高，AVFoundation产生的音频听起来很好

任何人都知道如何做到这一点？

import AVFoundation
import AudioToolBox

@IBAction func buttonOne(sender: UIButton) {

    var soundID: SystemSoundID = 0
    let soundURL = CFBundleCopyResourceURL(CFBundleGetMainBundle(), "C-note", "mp3", nil)
    AudioServicesCreateSystemSoundID(soundURL, &soundID)
    AudioServicesPlaySystemSound(soundID)

    let wSize : CGFloat = 20
    let hSize : CGFloat = 35

    let yPosition : CGFloat = 40

    let dot = UIImageView()
    dot.image = UIImage(named: "drop")
    dot.frame = CGRect(x: 33, y: yPosition, width: wSize, height: hSize)
    self.view.addSubview(dot)

    let duration = 0.7
    let delay = 0.0
    let options = UIViewAnimationOptions.CurveEaseIn

    UIView.animateWithDuration(duration, delay: delay, options: options, animations: {
        dot.frame = CGRect(x: 33, y: 360, width: wSize, height: hSize)
        }, completion: { animationFinished in
            dot.removeFromSuperview()
    })
}