Sorry. I just now saw this post.
I concur with Jeff and Jon. One flight without hearing protection would certainly be uncomfortable, but probably would not cause permanent damage. It's repetitive exposure to loud noises that causes the damage.
Also if the dogs are in carriers and you can't get to them during flight, it's unlikely you'll land with them still wearing them. The reason is that Mutt Muffs are not something most self-respecting dogs would want to wear. But most dogs, upon getting them off once, determine they were way happier with them on, and given the doggy empirical evidence, they don't take them off again.
So.... If I'm transporting dogs in the back seat, where my co-pilot can assist, we'll use Mutt Muffs on the pooch. If they're in a crate, I just apologize to the little guys for the loud conditions and remind them that they're almost home.
My dog, who flies with me alot, always wears his Mutt Muffs.