在 Android 6.0 或者之后的版本中要主动请求用户授权，具体方法，可以参考我之前写的这篇文章
《Android 6.0+ SMSManager 使用示例 发送短信》

实例的代码如下：

import android.app.Activity;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.Environment;
import android.view.KeyEvent;
import android.view.View;
import android.widget.Button;

import java.io.File;

public class MainActivity extends Activity implements View.OnClickListener {

    private static final String TAG = "MainActivity";

    private Button play;
    private Button pause;
    private Button stop;
    private MediaPlayer mediaPlayer = new MediaPlayer();

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

        play = findViewById(R.id.play);
        pause = findViewById(R.id.pause);
        stop = findViewById(R.id.stop);

        play.setOnClickListener(this);
        pause.setOnClickListener(this);
        stop.setOnClickListener(this);

        initMediaPlayer();
    }

    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            moveTaskToBack(true);
        }
        return super.onKeyUp(keyCode, event);
    }

    private void initMediaPlayer() {
        try {
            File file = new File(Environment.getExternalStorageDirectory(), "music.mp3");
            mediaPlayer.setDataSource(file.getPath());
            mediaPlayer.prepare();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.play:
                if (!mediaPlayer.isPlaying()) {
                    mediaPlayer.start();
                }
                break;
            case R.id.pause:
                if (mediaPlayer.isPlaying()) {
                    mediaPlayer.pause();
                }
                break;
            case R.id.stop:
                if (mediaPlayer.isPlaying()) {
                    mediaPlayer.reset();
                    initMediaPlayer();
                }
                break;
            default:
                break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (null != mediaPlayer) {
            mediaPlayer.stop();
            mediaPlayer.release();
        }
    }
}

本文参考自 《Android 第一行代码》