Please refer to Volume Library it will explain the volume.
Most coding knows what the value sent are so they don’t need to read it. Where below is likely the simplest way to read it and print it.
twobyte mp3_vol;
mp3_vol.word = MP3player.getVolume();
Serial.print(F("Right Volume = "));
Serial.println(mp3_vol.byte[0]);
Serial.print(F("Left Volume = "));
Serial.println(mp3_vol.byte[1]);
Note that the union for twobyte is already defined in the library.