docs: public void unload()
      {
          if (mAudioListener != null)
          {
              Log.d("SOUND", "unloading audio source %d %s", getSourceId(), getSoundFile());
              mAudioListener.getAudioEngine().unloadSoundFile(getSoundFile());
          }
          mSoundFile = null;
          mSourceId = GvrAudioEngine.INVALID_ID;
      }