I get I may get beat up on this but you are in a very bad position.
I am not an attorney and please do not interpret this a legal advice.
If you go to HR without a good case this person could harm you.
You could lose your position.
Once you you go formal he will go into a defense mode.
I would build a case. Microphone, witness, and email.
Lets leave legality of a microphone out of it.
If you know you have a microphone I suspect it is legal.
Legal in a court of law is not the same as legal in HR.
You play the tape to HR and that bell cannot be unrung.
So, if it is not a legal recording it cannot but used in a court of law but HR is not bound by rules of legal evidence.
Even if it is illegal (and I don't think it is).
He would have to file legal charges against you and that recording would have to be introduced into evidence.
He is not going to do that.