'Unspeakable' means 'inexpressibly terrible/bad', so this word has nothing to do with your subject. People who are unable to hear and speak were once called deaf and dumb, but these words are now out of favour (offensive to some), and are often replaced by ' hearing-impaired ' and ' speech-impaired '. From the American Heritage Dictionary: In reference to people who are unable to speak, mute and deaf-mute are now often considered objectionable.
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