The following video might help a few owners of spaniels, where the dog has gone deaf over time

The caption says 
"Some Cavalier King Charles Spaniels can have a condition arise called Primary Secrectory Otitis Media (PSOM). This is where the middle ear fails to drain properly and can fill with mucous and other material. The condition can cause deafness and terrible pain due to the ear being blocked and internal pressure."

Vet washing the inner ear of a cavalier king charles spaniel


Deaf dogs at dorsetdog.com