and… How to unstick software audio using Audio MIDI Setup

This has been a recurring problem with Macbook and Macbook Pros ever since Apple added the digital audio output to these portable Macs. The D/A converter is software controlled and occasionally will not bring audio back to the built-in speakers after a) headphones are unplugged from the computer, or b) an app takes over audio output and does not relinquish control after quitting.