Posttraumatic vocal disarticulation
From Halopedia, the Halo wiki
Posttraumatic vocal disarticulation is a Human condition involving the vocal chords of a human being. After a period of extreme stress or trauma, a person diagnosed with this "disease" is rendered completely unable to speak, even if there is no catastrophic injury to their actual vocal chords. It is unclear if this condition is physical, or if it is merely psychological.
Spartan-B091 is the only known person diagnosed with it in the Halo universe.
