Changes in vocalization in a senior dog are an indication for your dog to see a veterinarian for an examination. There are a number of different types of problems that lead to loss of voice in dogs.
Don't put your pet on a senior food or a "light" diet just because it is at a certain age. Diets formulated with the older pet in mind frequently contain fewer calories, 18% less protein and 4% to 6% less fiber.