How old is he? Voice changes in older cats can be caused by thyroid problems.
It can also be caused by viral or bacterial infection, cats can get strep throat the same as us and laryngitis, one of my boys lost his voice due to a bacterial infecion, he was very sick and we (vet included) were very scared for him but thankfully he pulled through with antibioticts.
I would suggest you take him to a vet, loss of voice could so easily be due to an infection and should be checked out by a vet.
I want to say he is around 7 or 8..he chews on plastic a lot too..but he has done that his whole life. I wanted to take him to the vet but I don't think I can afford too..and I'm scared I will lose him...
Call your vet and ask how much an exam is. Then ask if you can do a payment plan. Don't just sit there worrying - be proactive. Until you know what the costs might be your letting your imagination rule the situation.