For those unaware, firing guns without ear protection such as earplugs can cause severe hearing damage with long-term effects. Did the militaries of the 20th century provide for this? Were there any training or medical procedures to help soldiers deal with it? Are there any reported instances of such hearing damage being a problem among infantry?
Some hearing protection may have been used in the navy by gun crews, but in the armies, as far as I know, even artillerymen generally worked without any protection. I'm not sure that a study has ever been done on the percentages, but most every combat veteran of the war I have known was at least partially deaf, and some very deaf indeed. My own grandfather, who was an anti-aircraft gunner on a cruiser in the Pacific, reported ringing in his ears off-and-on for the reminder of his life; tinnitus, I should think. At the time, people probably didn't think much of it. Most hunters didn't wear hearing protection, and likely for the same reason as soldiers and sailors: before the shooting starts, you need to be able to hear at your very best, and there's rarely time to put earplugs in once it's gotten hot. Going deaf with age was if not normal, then very very common.
I have seen US GI WWII rubber earplugs, and used them for firing WWII small arms, and they worked quite well. But I don't know how often they were employed. There are still some WWII combat vets alive- they could be asked. And if the Army issued earplugs, you can bet they were mentioned in a Technical Manual.
Further question- If they weren't issued earplugs, what did they use? Any sane person would improvise if their ears were experiencing that kind of punishment.
Naval personnel may have worn ear plugs, but as far as I know through reading and classes most infantrymen (including artillery, tanks etc) did not carry them. There are many cases where soldiers come back from the wars and still hear the whistle of a mortar round. In the trenches sleep was hard to come by and it's one of the reasons why PTSD was so prevalent at the time. They were unprepared as well as the constant barrage of artillery fire . . . never knowing where it would land. Is that a bird in the distance or the reaper? It really didn't affect the people directly (maybe some hearing loss) but the main injuries are what was more paid attention to. It is probably one of the reasons why earplugs were issued but as guns got stronger so did the noise. Also there probably weren't any treatments for the soldiers exiting with deafness or severe anxiety problems, as PTSD wasn't diagnosed until the 1980's though there were military hospitals for the "insane" people, whom were probably suffering from PTSD.
Sigfried Sassoon has some amazing poems on the concept of war and the effect on the soldier.