public class MainActivity extends AppCompatActivity {
    Ringtone _defaultRingtone;
    Uri _uri;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        _defaultRingtone = RingtoneManager.getRingtone(MainActivity.this, Settings.System.DEFAULT_RINGTONE_URI);

//        _uri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_RINGTONE);

//        _defaultRingtone = RingtoneManager.getRingtone(MainActivity.this, _uri);

        configureButtons();

        _settingsContentObserver = new SettingsContentObserver(new Handler());
        this.getApplicationContext().getContentResolver().registerContentObserver(
                android.provider.Settings.System.CONTENT_URI, true,
                _settingsContentObserver);
    }

    void configureButtons() {
        Button btnStart = findViewById(R.id.btn_start);
        btnStart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                _defaultRingtone.play();
            }
        });

        Button btnStop = findViewById(R.id.btn_stop);
        btnStop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                _defaultRingtone.stop();
            }
        });
    }
}

void restartRingtone() {
    _defaultRingtone.stop();
    _defaultRingtone.play();
}

AudioManager am = (AudioManager) getSystemService(AUDIO_SERVICE);

switch (am.getRingerMode()) {
    case AudioManager.RINGER_MODE_SILENT:
    case AudioManager.RINGER_MODE_VIBRATE:
        restartRingtone();
        break;
}