Hello, my name is***** and I have over 20 years of veterinary experience. I'd like to help with your concerns about Greg's loss of voice.
Some upper respiratory infections, like Herpes or Calici Virus can cause a sore throat and a change in voice. That may be the only symptom or eventually they can also cause sneezing, eye or nose discharge, drooling, and sometimes joint inflammation as well. Upper respiratory infections in cats are contagious so if you have other cats you may wish to isolate him.
Some cats get sicker than others and young, healthy adult cats seem to tolerate them and fight them off better than very young kittens or older cats. An upper respiratory infection in cats is just like a cold in you and I, and these are commonly caused by a virus. These are easily caught by breathing in virus particles in the air from a sneeze or nasal or eye discharge.
You can help him feel better by adding warm water to his food to make it smell more (they don't eat if they cannot smell) as well as making it easier to chew and swallow.
Also the more fluids he gets the better. Offer tuna juice, low salt chicken broth, run the tap if he likes to drink out of the sink.
Take him into the bathroom with you if you run a hot bath or shower as the steam will soothe his sore throat and airways.
If his nose become very congested you can use sterile saline to loosen the thick mucous and remove it. He won't like it but it will help him breathe and be able to smell his food better. You can also use sterile saline to remove eye mucous if it accumulates.
If you notice mouth ulcers or a foul breath odor you can use Chlorhexidene oral rinses (like CET rinse) orally to keep him from getting a secondary bacterial infection and keep him more uncomfortable.
Some lethargy is understandable, let him rest as he needs rest to get better. If your cats normally go outdoors keep him (them) inside until they are back to their normal playful selves.
If he runs a high fever (more than 104F), has a green or yellow nasal or eye discharge, stops eating even with coaxing and clearing his nose and eyes, or starts coughing or having difficulty breathing then he needs a veterinary exam. Not an emergency as long as he is eating but soon if he isn't eating or has a green or yellow nasal or eye discharge.
Sometimes these upper airway infections turn into pneumonia so that's what we need to guard against. In most cases antibiotics aren't needed and can contribute to a decrease in appetite so I don't tend to prescribe them unless I feel there is evidence of a secondary bacterial component. These include a green or yellow eye or nasal discharge, evidence of pneumonia upon listening to their lungs or an infection that lingers beyond the normal 7 to 10 days.
I highly recommend testing him for feline leukemia and feline immunodeficiency viruses if his respiratory infection lingers. These immunosuppressive viruses will make a simple infection much worse as they stop the immune system from fighting infections the way it was designed to do. If he isn't much better in 7 to 10 days he needs a veterinary visit.
Here is a link if you want to read more about Calici virus: http://www.pet360.com/cat/health/calicivirus-in-cats/42gJj403b0idm8v6BjfOiA
Here's a link if you'd like to read more about Herpes virus in cats: http://www.icatcare.org:8080/advice/cat-health/feline-herpes-virus-fhv-infection
Other possible reasons for a change in voice include nasopharyngeal polyps, a laryngeal polyp or mass, or a foreign body caught in his pharynx and irritating his larynx.
Let me know if you have any further questions.