When I've done that I've noticed that I could only hear well straight ahead through the X-wing helmet, but that the helmet could also be a bit echo-y, making things sound more garbled in a crowded convention environment.
You can simply line the helmet with a foam or felt to absorb the sound.
I am going to say something that is not a popular opinion...
It is my humble opinion that there are no ear cups in the OT helmets.
Before I go any further, let me just state that I am in no way telling people they should remove their ear cups from their helmets. I personally have ear cups in my helmets.
At this point, people start whipping out images with red arrows showing me these tan/brown items protruding from the helmets. I am not debating that there is not something on the inside of the helmet, but what I am saying is that what you are seeing is some sort of helmet liner and not an ear cup.
Now I do not know if they fabricated or found something they could utilize. An ear cup in a flight helmet is something that presses toward and around the side of your head and creating a seal to keep noise in and other sounds out (which is the reason this post exists) but you would be able to see this in images of the movie scenes. First off, why would you want your actors to have hearing restrictions on the set when they do not need to. Second, the actors are wearing the Racal headsets (like the Falcon headset). The wires and mic are connected to the helmets. So there needs to be room in the helmet for the actor to wear them.
If the pilots were wearing earcups you would see them pressed against the face. They would also not extend to the bottom of the helmet as you see a lot of these photos illustrate. I think the intention was for the liner to fit well with in the helmet shell but in some cases the liner warps out and that is what people see and think are ear cups. You can see the liner much more clearly for Luke in Return of the Jedi than in previous movies. See image below. Note how far the liner goes to the bottom of the helmet, An ear cup does not go down that far. Note there is no microphone in this scene.