无法使用Android VideoView播放MP4

我想在一个Dialog盒子中播放视频，以响应ListViewitemclicked事件。在Dialog打开，我得到以下信息：

无法播放此视频

代码如下：

lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView < ? > parent, View view, int position, long id) {
        final Dialog dialog = new Dialog(aikiorder.this);
        dialog.setContentView(R.layout.aikiitemsample);
        dialog.setTitle(aikiNames[position]);
        Button b = (Button) dialog.findViewById(R.id.button2);
        b.setText(getString(R.string.closeVideoText) + " - " + aikiNames[position]);
        dialog.findViewById(R.id.button2).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                dialog.dismiss();
            }
        });

        final VideoView vv = (VideoView) dialog.findViewById(R.id.videoView2);
        MediaController mc = new MediaController(getApplicationContext());
        mc.setAnchorView(vv);
        mc.setMediaPlayer(vv);

        Uri video = Uri.parse("http://techslides.com/demos/sample-videos/small.mp4");
        vv.setVideoURI(video);
        vv.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
            @Override
            public void onPrepared(MediaPlayer mp) {
                mp.setLooping(true);
                vv.start();
            }
        });

        dialog.show();
    }
});