Ear-pro wasn't really a thing in either of the world wars. Some experimental stuff floated around, and people that were constantly exposed (gunnery instructors, for example) were sometimes given rudimentary protection.

Generally, the rank and file infantry/artillery didn't wear anything. They dealt with the hearing loss, like it or not. There's lots of anecdotal evidence of American GI's rigging up ear pro out of empty .45 ACP cases or cotton, but nothing formal was developed until near the war's end, and even then adoption didn't occur until postwar, when the Navy and Air Force started examining the effects of jet engines on personnel.

Here is a nice summary by the National Institute of Health on hearing loss studies through the ages for the US armed forces. Here is a VA claim for a navy vet who attributed his hearing loss to serving aboard a combat ship from '43-'46, had blamed it on that the whole time. So it's not like nobody realized that loud noises in combat were detrimental to hearing. Also, a report on the effectiveness of the V-51R ear plug that was developed out of that late-war study I mentioned earlier.

So yea, aside from empty pistol casings, some sort of cloth or fiber stuffed into the ears, or just holding your hands over them...not a lot of formal hearing loss prevention going on during the World Wars.

Also, this has been asked before, some decent answers cropped up here

This topic came up before, but it seemed to be inconclusive. I've also heard about using live ammunition as hearing protection which, while not being as good as proper earplugs, was better than nothing: Imgur. This is a photo from the 60s, but I imagine that ammo from the 1940s made equally effective ear protection.

Edit: whoops, gotta credit /u/Rittermeister for that response.