Android : Playing Image and audio simulteniously in android

I am trying to play audio and image concurrently in android...i have take one image-view and two buttons for go to back and go to next..when i click on next button and go on its working perfectly fine but as soon as i clicked on back button only audio file changes not the image. if i again clicked on back button then image changes but with another audio file..please help me out with this..thanks here is my code

    public class Main extends Activity{

Button btn_back,btn_play,btn_next;
ImageView img_view;
int current_img = 0;
int current_aud = 0;
MediaPlayer mp;

int[] images = {R.drawable.img1,R.drawable.img2,R.drawable.img3,R.drawable.img4,R.drawable.img5,
                R.drawable.img6,R.drawable.img7,R.drawable.img8,R.drawable.img9,R.drawable.img10,
                R.drawable.img11,R.drawable.img12};

int[] audio = {R.raw.aud,R.raw.a,R.raw.aud,R.raw.a,R.raw.aud,R.raw.a,R.raw.aud,R.raw.a,
                R.raw.aud,R.raw.a,R.raw.aud,R.raw.a};

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    btn_back = (Button)findViewById(R.id.button1);
    btn_play = (Button)findViewById(R.id.button2);
    btn_next = (Button)findViewById(R.id.button3);
    img_view = (ImageView)findViewById(R.id.imageView1);
    img_view.setImageResource(R.drawable.ic_launcher);
    mp =new MediaPlayer();

    btn_play.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
        mp.stop();  
        }
    });

    btn_next.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View arg0) {
            // TODO Auto-generated method stub
            current_img = current_img % images.length;
            img_view.setImageResource(images[current_img]);
            current_img ++;
        try {
            if(mp != null){
                if(mp.isPlaying()){
                    mp.stop();
                //  mp.release();
                }
            }


            current_aud = current_aud % audio.length;

            mp.reset();
            mp = MediaPlayer.create(Main.this, audio[current_aud]);
            mp.start();
             current_aud ++;





        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }

        }
    });

    btn_back.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub

            current_img = (current_img + images.length) % images.length;
            current_img --;
            img_view.setImageResource(images[current_img]);
            try {
                if(mp != null){
                    if(mp.isPlaying()){
                        mp.stop();
                        // mp.reset();
                    }
                }



                current_aud = (current_aud + audio.length)% audio.length;
                mp.reset();
                mp = MediaPlayer.create(Main.this, audio[current_aud]);
                current_aud --;
                mp.start();





            } catch (IllegalArgumentException e) {
                e.printStackTrace();
            } catch (IllegalStateException e) {
                e.printStackTrace();
            } catch (Exception e) {
                e.printStackTrace();
            }   
            }

    });