Can I be sued for interviewing with the media about immoral and illegal activities at my former employer?

You can be sued, although whether or not the suit would be successful, I certainly couldn't say without knowing a lot more about the facts.  Even if the lawsuit wasn't based on violation of some agreement that you signed, you could still be sued for defamation (slander).

What you say on the witness stand is privileged, meaning you can't be sued for that.  If the subject has become of enough public interest for the media to be asking you questions, the law gives you more protection for out of court statements, and you can't be found liable in the absence of actual malice.