I’m familiar with the loss of speech with dementia, but not loss of voice.
Perhaps there is a sore throat or other ailment that he isn’t able to articulate. Does he seem to respond negatively to loud noises or voices? As with any new symptom, I would contact his doctor. Let the doctor know when it started, and if it seems to be getting worse. Report if you are seeing any other symptoms or signs of pain (grimacing, frowning, not eating, changes in swallowing, restlessness, change in mood or sleep).