How to check a condition in android alwaysly? : i want to develop an app to change device audio status , if that is silent change it with a button to Normal and so on.

i Want to check Audio Status of device in my app always and if device is silent change my button text to silent and if it was normal , change it to normal.

here is my only class :

    public class MainActivity extends ActionBarActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final Button btn1= (Button) findViewById(R.id.btn1);
        final AudioManager audio = (AudioManager)getSystemService(Context.AUDIO_SERVICE);

        if (audio.getRingerMode()==AudioManager.RINGER_MODE_NORMAL)
        {
            btn1.setText("Normal");
        }
        else if (audio.getRingerMode()==AudioManager.RINGER_MODE_SILENT)
        {
            btn1.setText("Silent");
        }

        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                btn1.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        if (audio.getRingerMode()==AudioManager.RINGER_MODE_NORMAL)
                        {
                            audio.setRingerMode(AudioManager.RINGER_MODE_SILENT);
                            btn1.setText("Silent");
                        }
                        else if (audio.getRingerMode()==AudioManager.RINGER_MODE_SILENT)
                        {
                            audio.setRingerMode(AudioManager.RINGER_MODE_NORMAL);
                            btn1.setText("Normal");
                        }
                                          }
                });
            }
        });


how i can check this condition always in android ?