docs: public void setVolume(float volume)
      {
          // Save this in case this audio source is not being played yet
          mVolume = volume;
          if (isPlaying() && (getSourceId() != GvrAudioEngine.INVALID_ID))
          {
              // This will actually work only if the sound file is being played
              mAudioListener.getAudioEngine().setSoundVolume(getSourceId(), getVolume());
          }
      }