Deafness is definitely a problem in cavaliers and it can show up in quite young dogs.
The so called selective deafness described by so many owners, may be because often dogs can still hear certain high pitch sounds, so it will appear that sometimes they ignore the owners and sometimes they obey.
Two of my three boys are stone deaf. It can be a problem when we are on our walk & they decide to go their different ways; but our community garden is securely gated and they are old & slow now.
Fortunately Matthew still hears and they will usually follow him.
I use very extravagant hand signals to get them to come ( they soon learn if rewarded with a nice treat ) I touch them gently to wake them, and I smile, nod, and pat them to let them know they are loved.
Cavaliers......Tommy Tuppence 2000-2012 and a half share of Faith.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.