TweenMax Volume

Hello, I am trying to fade out some audio, using the code below, but I keep getting this error -

ReferenceError: Error #1069: Property volume not found on and there is no default value.
at gs::TweenLite/initTweenVals()
at gs::TweenMax/initTweenVals()

public function AudioStream(url, loop:Boolean = false)
_sound = new Sound();
var req:URLRequest = new URLRequest(url);
var context:SoundLoaderContext = new SoundLoaderContext(8000, true);
_sound.load(req, context);
if (loop) {, int.MAX_VALUE);
} else {;

public function stop()
{, 1, {volume:0, onComplete:stopAudio});