问从另一个活动控制Mediaplayer

EN

public void listenSwitch()
{
    s.setChecked(true); // On

    s.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {

            if (isChecked == false)
            {
                MainActivity.mp.stop(); // Works
            }
            else
            {
               // I use create(this, raw file) and then MainActivity.mp.start(); but error occurs
            }
        }
    });
}