Pass JSON to a Flash movie
As TandemAdam pointed out in his link you will need the AS3Corelib, which gives you access to the JSON class with the static methods JSON.deserialize() and JSON.serrialise().
If you're just communicating with the JavaScript on the page then you will have to continue to pass the JSON strings through ExternalInterface calls.
If you need to communicate with a server you can actually send the JSON strings directly to and from the server using the URLRequest object and URLLoader object.
This post should be helpful to you: ThanksMister - JSON (Lite)
It should point you in the right direction.