Definition of Personification
The illness is personified in "Speech Sounds," becoming an agent of evil or a bad character that exists solely to ruin lives:
The illness had stripped her, killing her children one by one, killing her husband, her sister, her parents...The illness, if it was an illness, had cut even the living off from one another.
By allowing the illness to "strip" Rye and "kill" her family, Butler gives it more agency and identity. However, the frustrating thing about the illness is that it cannot be identified. Rye does not even know for sure "if it was an illness," making its concrete effects all the more upsetting:
The illness had played with them, taking away, she suspected, what each valued most.
Rye can direct her rage toward a personified illness but not toward any tangible physical form. What appears to be a game for the illness, who plays with its victims, is very real for Rye.
Rye's hope at the end of the story relies on the illness choosing not to "play" with the two children:
If the illness let these children alone, she could keep them alive.
Though only in her mind, Rye makes a plea to this character or personified entity, begging it to spare the children that she believes she can save. Appealing to the illness's supposed power of choice is another example of its agency.