Android java JSON parsing 'null' as output

We have been using this code in our application to fetch data from an online database, but whatever we tried, it keeps saying "null" as an output by album and duration. It could be a very small detail, but since we are beginning developers, we don't really know a lot about JSON. Has anyone got any suggestions?

package com.example.androidhive;

import java.util.ArrayList;
import java.util.List;

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.text.Html;
import android.util.Log;
import android.widget.TextView;

import com.example.androidhive.helper.AlertDialogManager;
import com.example.androidhive.helper.ConnectionDetector;
import com.example.androidhive.helper.JSONParser;

public class SingleTrackActivity extends Activity {
    // Connection detector
    ConnectionDetector cd;

    // Alert dialog manager
    AlertDialogManager alert = new AlertDialogManager();

    // Progress Dialog
    private ProgressDialog pDialog;

    // Creating JSON Parser object
    JSONParser jsonParser = new JSONParser();

    // tracks JSONArray
    JSONArray albums = null;

    // Album id
    String album_id = null;
    String song_id = null;

    String album_name, song_name, duration;

    // single song JSON url
    // GET parameters album, song
    private static final String URL_SONG = "http://api.androidhive.info/songs/track.php";

    // ALL JSON node names
    private static final String TAG_NAME = "name";
    private static final String TAG_DURATION = "duration";
    private static final String TAG_ALBUM = "album";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_single_track);

        cd = new ConnectionDetector(getApplicationContext());

        // Check if Internet present
        if (!cd.isConnectingToInternet()) {
            // Internet Connection is not present
            alert.showAlertDialog(SingleTrackActivity.this, "Internet Connection Error",
                    "Please connect to working Internet connection", false);
            // stop executing code by return
            return;
        }

        // Get album id, song id
        Intent i = getIntent();
        album_id = i.getStringExtra("album_id");
        song_id = i.getStringExtra("song_id");

        // calling background thread
        new LoadSingleTrack().execute();
    }

    /**
     * Background Async Task to get single song information
     * */
    class LoadSingleTrack extends AsyncTask<String, String, String> {

        /**
         * Before starting background thread Show Progress Dialog
         * */
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            pDialog = new ProgressDialog(SingleTrackActivity.this);
            pDialog.setMessage("Loading song ...");
            pDialog.setIndeterminate(false);
            pDialog.setCancelable(false);
            pDialog.show();
        }

        /**
         * getting song json and parsing
         * */
        protected String doInBackground(String... args) {
            // Building Parameters
            List<NameValuePair> params = new ArrayList<NameValuePair>();

            // post album id, song id as GET parameters
            params.add(new BasicNameValuePair("album", album_id));
            params.add(new BasicNameValuePair("song", song_id));

            // getting JSON string from URL
            String json = jsonParser.makeHttpRequest(URL_SONG, "GET",
                    params);

            // Check your log cat for JSON reponse
            Log.d("Single Track JSON: ", json);

            try {
                JSONObject jObj = new JSONObject(json);
                if(jObj != null){
                    song_name = jObj.getString(TAG_NAME);
                    album_name = jObj.getString(TAG_ALBUM);
                    duration = jObj.getString(TAG_DURATION);                    
                }           

            } catch (JSONException e) {
                e.printStackTrace();
            }

            return null;
        }

        /**
         * After completing background task Dismiss the progress dialog
         * **/
        protected void onPostExecute(String file_url) {
            // dismiss the dialog after getting song information
            pDialog.dismiss();

            // updating UI from Background Thread
            runOnUiThread(new Runnable() {
                public void run() {

                    TextView txt_song_name = (TextView) findViewById(R.id.song_title);
                    TextView txt_album_name = (TextView) findViewById(R.id.album_name);
                    TextView txt_duration = (TextView) findViewById(R.id.duration);

                    // displaying song data in view
                    txt_song_name.setText(song_name);
                    txt_album_name.setText(Html.fromHtml("<b>Album:</b> " + album_name));
                    txt_duration.setText(Html.fromHtml("<b>Duration:</b> " + duration));

                    // Change Activity Title with Song title
                    setTitle(song_name);
                }
            });

        }

    }
}