یه مشکل کوچولو در پخش موزیک

[javadi3d]

در کد زیر وقتی موزیک دانلود شد هنگامی که روی play کلیک میکنی فایل پخش نمیشه ولی وقتی از اکتیویتی خارج و دوباره داخل آن می شوی مشکل حل میشود و موزیک پلی میشود

چه چیزی باید به کد اضافه کنم؟
لطفا کمکم کنید






public class DoaMatn1 extends Activity implements OnClickListener {
MediaPlayer mp;
ImageButton btndowndoa;
ImageButton btnplaydoa;
SeekBar seek_bar;
Handler seekHandler = new Handler();
private ProgressDialog pDialog;
public static final int progress_bar_type = 0;
private static String file_url = "http://www.uplooder.net/f/tl/477caca725f67b4ee822aa5666d9afb2/basem_tavas.mp3";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.doamatn);
WebView wv = (WebView) findViewById(R.id.wvdoa);
wv.setBackgroundColor(0x00000000);
wv.setHorizontalScrollBarEnabled(false);
wv.loadUrl("file:///android_asset/doatavasol.html");
WebSettings webSettings = wv.getSettings();
webSettings.setDefaultFontSize(20);
mp = MediaPlayer.create(this,Uri.fromFile(audioFile));
btnplaydoa = (ImageButton) findViewById(R.id.btnplaydoa);
btnplaydoa.setOnClickListener(this);
btndowndoa = (ImageButton) findViewById(R.id.btndowndoa);
btndowndoa.setOnClickListener(this); getInit();
seekUpdation();
} Intent intent = new Intent(android.content.Intent.ACTION_VIEW);
File sdcard = Environment.getExternalStorageDirectory();
File audioFile = new File(sdcard.getPath() + "/music/basem-tavasol.mp3");
public void getInit() {
if(audioFile.exists())
{
seek_bar = (SeekBar) findViewById(R.id.sbdoa);
seek_bar.setMax(mp.getDuration());
}}
Runnable run = new Runnable() {

@Override
public void run() {
seekUpdation();
}
};

public void seekUpdation() {
if(audioFile.exists())
{
seek_bar.setProgress(mp.getCurrentPosition());
seekHandler.postDelayed(run, 1000);

}

}

@Override
public void onClick(View v) {
switch(v.getId())
{
case R.id.btnplaydoa :
if(audioFile.exists())
{

if(mp!=null)
{
if(mp.isPlaying())
{
mp.pause();
btnplaydoa.setImageResource(R.drawable.play);
}
else
{
mp.start();
btnplaydoa.setImageResource(R.drawable.puse);
}
}
}
else
{
Builder alert = new AlertDialog.Builder( DoaMatn1.this);
alert.setTitle("Alert");
alert.setMessage("فایل دانلود نشده است");
alert.setPositiveButton("OK", null);
alert.show();
}
break;

case R.id.btndowndoa :

if(!new File(Environment.getExternalStorageDirectory().toS tring() + "/music/basem-tavasol.mp3").exists())
new DownloadFileFromURL().execute(file_url);

else
{
AlertDialog.Builder dialogBuilder = new AlertDialog.Builder(DoaMatn1.this);
dialogBuilder.setMessage("This file has already been downloaded");
AlertDialog dialog = dialogBuilder.create();
dialog.show();
}
break;
}
}



@Override
public void onBackPressed() {
if( mp != null && mp.isPlaying() ) {
mp.stop();
}
super.onBackPressed();
}

@Override
protected Dialog onCreateDialog(int id) {
switch (id) {
case progress_bar_type:
pDialog = new ProgressDialog(this);
pDialog.setMessage("Downloading file. Please wait...");
pDialog.setIndeterminate(false);
pDialog.setMax(100);
pDialog.setProgressStyle(ProgressDialog.STYLE_HORI ZONTAL);
pDialog.setCancelable(true);
pDialog.show();
return pDialog;
default:
return null;
}
}
class DownloadFileFromURL extends AsyncTask<String, String, String> {

/**
* Before starting background thread
* Show Progress Bar Dialog
* */
@Override
protected void onPreExecute() {
super.onPreExecute();
showDialog(progress_bar_type);
}

/**
* Downloading file in background thread
* */
@Override
protected String doInBackground(String... f_url) {
int count;
try {
URL url = new URL(f_url[0]);
URLConnection conection = url.openConnection();
conection.connect();
// getting file length
int lenghtOfFile = conection.getContentLength();

// input stream to read file - with 8k buffer
InputStream input = new BufferedInputStream(url.openStream(), 8192);

// Output stream to write file
OutputStream output = new FileOutputStream("/sdcard/music/basem-tavasol.mp3");

byte data[] = new byte[1024];

long total = 0;

while ((count = input.read(data)) != -1) {
total += count;
// publishing the progress....
// After this onProgressUpdate will be called
publishProgress(""+(int)((total*100)/lenghtOfFile));

// writing data to file
output.write(data, 0, count);
}

// flushing output
output.flush();

// closing streams
output.close();
input.close();

} catch (Exception e) {
Log.e("Error: ", e.getMessage());
}

return null;
}

/**
* Updating progress bar
* */
protected void onProgressUpdate(String... progress) {
// setting progress percentage
pDialog.setProgress(Integer.parseInt(progress[0]));
}

/**
* After completing background task
* Dismiss the progress dialog
* **/
@Override
protected void onPostExecute(String file_url) {
// dismiss the dialog after the file was downloaded
dismissDialog(progress_bar_type);


}

}}