Nope no Flounder or Croakers in PA...but the fish that you find on the sound side are generally bass, sunfish and such...at least up around the Duck / Corolla area. It is generally freshwater that far north - the inlet is quite a ways south of there before you hit the salt water. The area might be considered slightly brackish at best. It is my understanding that these are the fish that you will catch in that area from what bthe local shops told me. if you cross over the banks to the ocean side then you are talking flounder, croaker, cobia, mackerel, etc.
When I bought my license last year they told me I would need a separate inland license to fish in the sound. So I just opted for the Coastal Recreational Fishing License (CRFL) that only cost ya $10 for ten days or as Dale mentioned $30 for a year. Or if you don't want to pay anything to fish head to one of the piers. You don't need a license at all to fish from the piers, they have a blanket license that covers anybody fishing from the pier.
Have fun fishing I'll let ya know how we make out this week
Steve