Dogs are more of a noise hazard than something that should be bothered by
noise. :-)

Anything that can stand their barking is not likely to suffer hearing loss
from a mere airplane. If you are still concerned about it, you can clamp
blocks of high density foam over their ears using a strap of some kind. I
don't think our Scottie would tolerate this for a minute, but, well, she's a
Scottie. She normally just sleeps and passes gas during a flight.

Dogs' heads vary in shape so much that it would be almost impossible to
produce commercially viable hearing protection. The biggest market would be
hunting dogs, since only a few kinds of dogs are used and there are a lot of
hunters. Lots of hunting and gun dog shops carry hearing protection for dog
owners, but none for the dogs.