A couple shots in the dark:
Try adding 2 seconds of silence to the end of the offending slide and see if that helps/hurts/does nothing.
Try editing that audio file in another program (Audacity is free) and add the desired .5 seconds of silence there and then re-import the audio. See if that makes a difference.
Thanks for the ideas. Increasing the silence in Presenter didn't fix it, but ading the .5 seconds of silence in an outside editor and re-importing did it! Yay!
Now I'm off to make that change to eight courses. Boooo.
While I don't know that it is a good solution, I'm glad it is a solution.
I'll see if I can bring this to the attention of Adobe and we'll see if they can help identify the cause.
Indeed, it's not ideal, but it does seem to be getting the job done. Thanks for your assistance!