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.
Loss of barking ability may be due to physical problems. For example, an older dog may get a cancer or inflammation affecting the vocal cords or the area/tissues surrounding them. This may lead to discomfort when the dog barks, or an inability to make a barking sound because the flexibility of the cords is lost, or pressure on them leads to loss of function. Changes in the brain due to cancer or inflammation or general medical conditions (hormonal conditions, deafness) may also lead to changes in normal vocalization patterns. If the loss of voice is short lived, it might be due to inflammation in the throat caused by a bacterial infection (ex. strep throat).
Another common reason for change in vocalization is behavior-related. Senior dogs may be affected by what used to be referred to as senility or senile dementia, but is now termed cognitive dysfunction syndrome. Though loss of interest in barking/howling could be the only sign, more frequently more than one of the following types of signs is also noted
Change in sleep patterns
Loss of training (will not come when called, house soiling)
Loss of interest in family
Change of appetite
If dogs experience reduced mental capacity due to cognitive dysfunction syndrome, there is medication and diet therapy now available which can stabilize the condition, at least in the short term.