We see humans who have a possibly incurable virus that makes them unable to speak. Watching them straining in vain to speak was painful for me, because my dad has advanced Parkinson's and must struggle to say even one word.
Cesaer’s wife is fridged. She has no role except for having the protagonist’s sons in the previous film and being his motive for revenge after she’s brutally murdered in this one.
Other mutes are found buried after being shot. One survived and is “euthanized” by the protagonist (it seems to be because he’s in pain from his wounds rather than because of his disability, though). The villain commits s*icide after becoming mute. “Speech = intelligence” rhetoric and the belief that the spread of an illness that causes muteness among humans is nature establishing apes as “superior.” Maurice learns to talk after only signing in the past two films which ties into “curing disability” rhetoric.