Should I sue my coworkers for Defamation of Character and Slander?

As soon as my 90 day probation period ended at my new job and I was allowed to start using the personal, vacation and sick time that I accumulated. I spoke with my superior and asked if on slow days if it would be ok if I used some of that time as opposed to sitting at my desk doing nothing. My superior not only gave permission but said it was a good idea because when we started getting busy, it would be doubtful that any requested time off would be approved. So on occasion I would either send an email requesting that I use a few hours or asked directly face to face and each time I was approved. However, I was not obligated to inform my coworkers that I was leaving for the day and they started spreading it around that I was stealing time from my employer. When I caught wind of this I sent an email asking them to meet with me. They each accepted (There were 2) who not only didn't deny they said it, but actually made the accusation to my face. Unfortunately for them I am well versed in the law for my state that allows me to record conversations under State and Federal guidlines as I live in a State that has "One Party Consent" laws. Now I know I can use these recordings legally against them and even hope they lie on the stand when I ask them if they made the accusations so they can be charged with purgering themselves as well. Howvwe I do need to know if I am legally bound to present the recordings to the defense prior to trial or can I just hold onto them and only call them into evidence if I catch them trying to lie? Can't this type of evidence be used as a type of rebuttal witness? Thanks in advance for any answers you can provide.