Hi notsleepingwell,
You asked "Do I let him hear it?". In my opinion, no. I guess I believe that too many kids nowadays already have way too many issues to deal with than they need - especially if addiction and substance abuse have been in the picture.
Ultimately, you will make this decision, but before you do, I hope you consider your reasons for letting him listen to it. Do you want him to listen to it to prove what an a** the ex boyfriend is? Or would you do it to prove that you are are not deserving of the hostile treatment they have both given you?
I can understand where you are coming from. And for me, years ago, the answer was yes to both. I had tapes too. My son was younger than your grandson at the time, but I felt a need to SHOW him what a miserable, angry man his father was. But I didn't. I made sure my son was safe and I talked with him when his father did stupid and inconsiderate things. I listened -- which was hard at times. And you know what? He found out all on his own, without any help from me.
Hope things work out for you -- whatever you decide.