Formerly deceased? You mean that he merely "was" being deceased? :-> (Okay, I'm kidding with you there. I know what you are meaning.)
It could be that his recordings are made in advance. It's a normal thing to do when producing a series for broadcast and caters for the unexpected events if you have a buffer of recorded material that hasn't been broadcast yet. If it is teaching material, then these can also be rotated in the same way that you will see repeats of "Friends" and other programmes.
Then there are preachers like Chuck Swindoll whose radio broadcasts are from material they preached and recorded in the 70s and are still as fresh and valid today. (Hairstyles and clothing fashion don't carry over to radio.)
Chuck Swindoll is excellent and perfectly legit for Bible teaching material. I've also listened to quite a few recordings of
D. James Kennedy's "Truths That Transform" radio programme and was very impressed with the depth of knowledge and quality of the content.
John