Definition of Metaphor
After Rye and Obsidian have sex, Rye thinks about the children she lost versus the children growing up in a chaotic, post-pandemic world. In a telling simile, she laments the way society has come apart:
Today's children gathered books as well as wood to be burned as fuel. They ran through the street chasing one another and hooting like chimpanzees. They had no future. They were now all they would ever be.
While this connection between humans without language and animals (or animal sounds) is more subtle earlier in the story, here Butler makes the link clear. Without language, humans have lost civilization; instead of talking to each other, they can only "hoot," making the titular "Speech Sounds" that fail to resemble intelligible speech. The future becomes unthinkable when humans revert to a more primal form.
Later, Rye turns this simile into a metaphor when she remarks that the two now motherless children "would grow up to be hairless chimps." Without a mother and a structured society to guide their development, these children can only grow into animals. Going from simile ("like chimpanzees") to metaphor ("to be hairless chimps") turns a hypothetical into a reality or a comparison into a conclusion.
When Rye finds out the children can speak, however, she retracts this dehumanizing metaphor. Now that Rye knows she can communicate with the children, she can imagine a future for them (and for herself). They are no longer irredeemable but teachable, giving Rye hope for humanity.