There are a number of things that can cause a loss of voice in a cat.
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
If your vet did not see anything on the physical exam, they would have to do further diagnostics to figure out the cause. Sorry : (
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 icecream might help as well. I am talking a teaspoon not a scoop : )