Before replacing the logic board or speaker, plug the microphone into the headphone port a couple of times, and then pull it back out.
The headphone port isn't designed for the longer connector used on the PlainTalk microphone. If the microphone connector is accidentally inserted into the headphone port by mistake, it's may jam the headphone port in a closed position. This may cause the system think there are headphones or external speakers connected and disable sound through the internal speaker.
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