package com.turtle920.androidaudioprocess;

import android.media.MediaPlayer;

import android.os.Environment;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.util.Log;

import android.view.View;

import android.widget.Button;

import java.io.IOException;

public class MainActivity extends AppCompatActivity {

    MediaPlayer mp = new MediaPlayer();

    float leftVol=0.5f, rightVol=0.5f;

    String song = Environment.getExternalStorageDirectory().getAbsolutePath() + "/Music/tf.mp3";

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        try {

            mp.setDataSource(song);

            mp.prepare();

            mp.setVolume(leftVol,rightVol);

        } catch (Exception e) {

            Log.e("DEBUG", "" + e.toString());

            e.printStackTrace();

        }

        Button button1 = (Button) findViewById(R.id.button_start);

        button1.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                mp.start();

            }

        });

        Button button2 = (Button) findViewById(R.id.button_pause);

        button2.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                mp.pause();

            }

        });

        Button button3 = (Button) findViewById(R.id.button_replay);

        button3.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                mp.stop();

                try {

                    mp.prepare();

                } catch (IOException e) {

                    e.printStackTrace();

                }

            }

        });

        Button button4 = (Button)findViewById(R.id.button_l);

        button4.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                if (leftVol+0.05f<=1.0f) leftVol+=0.05f;

                if (rightVol-0.05f>=0f) rightVol-=0.05f;

                Log.e("DEBUG",""+leftVol+" "+rightVol);

                mp.setVolume(leftVol, rightVol);

            }

        });

        Button button5 = (Button)findViewById(R.id.button_r);

        button5.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                if (leftVol-0.05f>=0f) leftVol-=0.05f;

                if (rightVol+0.05f<=1.0f) rightVol+=0.05f;

                Log.e("DEBUG",""+leftVol+" "+rightVol);

                mp.setVolume(leftVol, rightVol);

            }

        });

    }

}

 <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>