音声が再生されません

先日、音声再生のプログラムを記述したのですが、音声が再生されません。ご指摘の程、よろしくお願いします。

import UIKit
import AVFoundation

class ViewController: UIViewController {

    var player:AVAudioPlayer?

    let sound_data = NSURL(fileURLWithPath: NSBundle.mainBundle().pathForResource("sample", ofType: "mp3")!)

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        do {
            let player = try AVAudioPlayer(contentsOfURL: sound_data)
            player.play()
        }
        catch let error as NSError {
            print(error)
        }

        player?.prepareToPlay()

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @IBAction func play(sender: UIButton) {
        player?.play()
    }

    @IBAction func pause(sender: UIButton) {
        player?.pause()
    }

    @IBAction func stop(sender: UIButton) {
        player?.stop()
        player?.currentTime = 0

    }
}