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The purr is controlled by nerves that innervate the diaphragm muscle.
The voice is controlled by the "voice box" located in the throat.
There are a number of things that can cause a loss of voice in a cat. You most likely will have to take him to the vet for a diagnosis if the voice does not return in the next day or so.
1. A viral infection by causative agents such as the feline herpes virus or feline calici virus. These can cause inflammation of the throat and produce a laryngitis. You may, or may not, see other upper respiratory signs with these viruses as well. Bacterial infections can produce similar signs.
2. A foreign body could be stuck in the throat.
3. There could be a polyp, or a mass, in the throat.
4. Laryngeal paralysis which could be caused by a problem with the nerves signals going to that area or due to inflammation.
5. Too much talking. An example of this would be if a new cat kept showing up outside the window and your cat kept telling it to get lost.
6. Hyperthyroidism www.VeterinaryPartner.com/Content.plx?P=A&A=516
7. Hyopkalemia which means low blood potassium
So if he is not better in a day or so, he needs to go to the vet for further testing.
If he does not have a sensitive stomach, give him a little warm chicken broth and see if that soothes his throat.
A spoonful of vanilla ice cream might help as well. I am talking a teaspoon not a scoop : )