Thank you. This is the part that I had forgotten - specifically where to do this expiration. It's done via the "folder" named 'AEP Audiences' in our environment.

Yes. I needed to look in ACCv8 at the list of audiences imported from AEP in the "AEP Audiences" folder/list and set the expiration date there. Thank you to everyone for your input.

We are actively trying to manage the burgeoning number of audiences in AEP and then "downstream" in Campaign v8. I've been told to "expire" any no-longer-necessary audiences in both tools, but I'm not sure how to do this.