A legal person this side. There isnโt much you can do. The call recording doesnโt hold much evidentiary value. The victim will never testify. There wonโt be any medical evidence as well. There isnโt an eyewitness either. So basically it would be just your word against theirs. The girl would straight up deny it. So basically, there isnโt anything you can do.
Although you can look into using this fact in your divorce proceedings. The fact that your husband is having an affair with an underage girl, will definitely help the court side with you in your case. Good luck.