It's not clear whether you're calling AS from JS or vice versa. If AS is calling JS, then you can simply pass parameters in the ExternalInterface calls.

Code:
ExternalInterface.call("someJSFunction", currentTrack, isPlaying);
If you are calling AS from JS, you could create and expose getter functions for your properties
Code:
private function getCurrentTrack():String{
  return currentTrack;
}
private function getIsPlaying():Boolean{
  return isPlaying;
}
//...
ExternalInterface.addCallback("getCurrentTrack", getCurrentTrack);
ExternalInterface.addCallback("getIsPlaying", getIsPlaying);
Javascript can then call those methods to get your values.