Fixed code by Jong

package com.test.brzoracunanje;

import android.app.Activity;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.KeyEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

public class PocetnaActivity extends Activity implements OnClickListener {


    TextView tvNaslov;
    ImageView imgLogo;
    Button btnStart, btnInstrukcije, btnRank, btnIzlaz;
    //MediaPlayer mMediaPlayer;
    BackgroundSound mBackgroundSound;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.pocetna);

        tvNaslov = (TextView) findViewById(R.id.tvRacunalica);
        imgLogo = (ImageView) findViewById(R.id.imageView1);
        btnStart = (Button) findViewById(R.id.btnStart);
        btnInstrukcije = (Button) findViewById(R.id.btnInstrukcije);
        btnRank = (Button) findViewById(R.id.btnRank);
        btnIzlaz = (Button) findViewById(R.id.btnIzlaz);

        btnStart.setOnClickListener(this);
        btnRank.setOnClickListener(this);
        btnInstrukcije.setOnClickListener(this);
        btnIzlaz.setOnClickListener(this);

        //Intent svc=new Intent(this, BackgroundSoundService.class);
        //startService(svc);

    }

    public void onResume() {
        super.onResume();
        mBackgroundSound = new BackgroundSound();
         mBackgroundSound.execute(null);

        }

        public void onPause() {
            super.onPause();
            mBackgroundSound.cancel(true);
        }


    @Override
    public void onClick(View v) {

        switch (v.getId()) {

        case R.id.btnStart:

            Intent intent = new Intent(this, BrzoRacunanjeActivity.class);

            startActivity(intent);

            break;

        case R.id.btnInstrukcije:

            Intent a = new Intent(this, InstrukcijeActivity.class);

            startActivity(a);

            break;

        case R.id.btnRank:

            Intent b = new Intent(this, RankPrikazActivity.class);

            startActivity(b);
            break;

        case R.id.btnIzlaz:

            Intent c = new Intent(this, UcenjeActivity.class);

            startActivity(c);
            break;

        }

    }



    public class BackgroundSound extends AsyncTask<Void, Void, Void> {
            MediaPlayer mMediaPlayer;
        protected void onPreExecute() {
        mMediaPlayer = MediaPlayer.create(PocetnaActivity.this, R.raw.test_cbr);
        }
        protected Void doInBackground(Void... params) {
            mMediaPlayer.setLooping(true); // Set looping
            mMediaPlayer.setVolume(100,100);
            mMediaPlayer.start();
            return null;
        }
        protected void onCancelled(Void v) {
        mMediaPlayer.stop();
        mMediaPlayer.release();
        }


    }


}