I've been debugging all day and found a few recent threads about it. Sounds like another Sierra bug. Something's definitely wrong with speechsynthesisd:

OpenRadar: NSSpeechSynthesizer deadlocks when writing to a file and calling stopSpeaking

StackOverflow: Ideas of how to fix this deadlock in NSSpeechSynthesizer?

In reply to Re^4: Module mysteriously stopped working by Anonymous Monk
in thread Module mysteriously stopped working by Anonymous Monk

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