code: public void fadeIn (float interval)
      {
          if (_state != AL10.AL_PLAYING) {
              play();
          }
          _source.setGain(0f);
          _fadeMode = FadeMode.IN;
          _fadeInterval = interval;
          _fadeElapsed = 0f;
      }
docs: Fades this stream in over the specified interval.  If the stream isn't playing, it will be
  started.