Android側のソースにsetVolumeControlStreamを書いてあればOK。

package hoge;
 
import org.cocos2dx.lib.Cocos2dxActivity;
import android.os.Bundle;
import android.media.AudioManager;
 
public class Hoge extends Cocos2dxActivity {
 
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setVolumeControlStream(AudioManager.STREAM_MUSIC);
    }
    
    static {
         System.loadLibrary("cocos2dcpp");
    }
}