Android audio player
You could let your class implement OnClickListener

public class MainActivity extends Activity implements OnClickListener{
    ...
}


and then use the onClickfunction like this:

public void onClick(View v)
{
    switch (v.getId())
    {
        case R.id.button1:
            mp.start();  
            break;
        case R.id.button2:
            mp.pause();  
            break;
        case R.id.button3:
            mp.stop();  
            break;
        default:
            break;
    }
}


That way you have all actions that happen on a press of a button in the same place in your program.

You should also change the ID to something more meaningful, eg. play, pause, stop.
This makes your code far more maintainable and easier to understand which Button does what just by skimming the code of the MainActivity(which you also might want to rename to something like MediaPlayerActivity incase you decide to add additional Activity classes, for example to select a new song.

Additionally I would advise you to change the visibility of the Buttons to private, rather than default.