How to change the global music volume on Android

MainActivity.java

package com.fiskur.wearvolume;

import android.app.Activity;
import android.content.Context;
import android.media.AudioManager;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener {
	
	private TextView mVolumeLabel;
	private Button mVolumeUpButton;
	private Button mVolumeDownButton;
	private AudioManager mAudioManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        mVolumeLabel = (TextView) findViewById(R.id.volume_label);
        
        mVolumeUpButton = (Button) findViewById(R.id.volume_up_button);
        mVolumeUpButton.setOnClickListener(this);
        
        mVolumeDownButton = (Button) findViewById(R.id.volume_down_button);
        mVolumeDownButton.setOnClickListener(this);
        
        mAudioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE);
    }

	@Override
	public void onClick(View view) {
		int maxVolumeIndex = mAudioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC);
		int volumeIndex = mAudioManager.getStreamVolume(AudioManager.STREAM_MUSIC);
		if(mVolumeUpButton == view){
			if(volumeIndex < maxVolumeIndex)volumeIndex++;
			mAudioManager.setStreamVolume(AudioManager.STREAM_MUSIC, volumeIndex, 0);
			
		}else if(mVolumeDownButton == view){
			if(volumeIndex > 0)volumeIndex--;
			mAudioManager.setStreamVolume(AudioManager.STREAM_MUSIC, volumeIndex, 0);
		}
		
		mVolumeLabel.setText("Max Volume Index: " + maxVolumeIndex + " Volume Index: " + volumeIndex);
	}
}