I was involved in a shooting that greatly exacerbated the tinnitus that I already had from playing bass, and I used to watch a Youtube channel run by an Army Ranger who had a traumatic brain injury from firing a rocket with no hearing protection, so it always kind of blows my mind that soldiers spent decades if not centuries fighting amidst automatic weapons and artillery with ostensibly no hearing protection. Did everyone just kind of deal with the massive hearing loss? I see soldiers whispering commands to each other in movies when they're fighting under stealth conditions, was that actually feasible for soldiers who had been through a lot of combat? I remember at the time my brain tricking me into thinking that the shots weren't nearly as loud as they actually were, but I can imagine my hearing would deteriorate extremely quickly with any sort of prolonged exposure.
This is an older answer I wrote that focuses on US military hearing protection (or lack thereof) in World War II, and developments onwards to the turn of the century, so a bit broader than your question, but relevant enough in its focus on the lessons from that period that I'll repost it here:
Hearing loss has been a negative side-effect of military service at least since the rise of firearms, with the Frenchman Ambroise Paré writing about the phenomenon in the 1500s. As far as the US military goes, the government began to acknowledge that hearing loss was a disability after the Civil War, which saw 1/3 of veterans receiving a diagnosis indicating some level of hearing loss (and possibly more who simply weren't diagnoses). Up through the mid-1940s though, just because it was acknowledged that a) guns were loud and b) loud things damage your hearing, very little was done to actually solve this problem.
Certainly on the individual level, that isn't to say servicemen wouldn't have acted on their own initiative, but options were limited as no professional ear protection was available. The simplest option was a finger in the ear, such as during artillery barrage - whether firing or receiving. Balls of cotton in the ear was also a very common option, found in countless recollections of the World Wars and earlier, and even recommended by professionals, who suggested wetting the cotton with petroleum jelly to insert. Purpose made earplugs also did exist in the time, but not part of any standard issue kit. And although any pride of cloth would do in a pinch, apparently reports of soldiers who would literally stuff dirt in their ears during an artillery barrage were not to uncommon.
But even taking account of what meager options existed, use by military men of the period could often be lackluster, and studies often note how the macho culture of the military provided a real pushback against this, as attempting to protect yourself from loud noises was viewed as unmanly. The first official recommendation came from the Army's Armored Medical Research Laboratory at Fort Knox, which released a study in 1944 recommending that hearing-protection be provided to men regularly involved in tank and artillery gunnery. The result was to make available the V-51R ear plug, but gun crews were only recommended to use them, not required. And even with them, their effect was marginal at best.
Although treatment for hearing loss also started to see improvement in this time, with both the Army and Navy opening Aural Rehabilitation Centers during the war, the original focus was to increase manpower by treating those who were on the cusp of military requirements for hearing capabilities. It would only be as the war started to wind up, and veterans began returning home with serious hearing loss that treatment of veterans increased. The Army Audiology and Speech Correction Center at Walter Reed consolidated several smaller groups in 1946, and TB MED 195 "The Army's Audiology and Speech correction Program for the Deafened" was published in 1947, but hearing rehabilitation was not a priority compared to other injuries, and much of the progress in treatment came from universities that opened audiology clinics under government sponsorship.
That all said, while it is very clear that there were significant negative effects on the hearing of World War II veterans, it is hard to say exactly how extensive it might be as records on that specific issue apparently are lacking. To quote from Humes:
Because large numbers of people have served in the military since World War II, the total number who experienced noise-induced hearing loss by the time their military service ended may be substantial, but the available data provide no basis for a valid estimate of the number. Neither was it possible to estimate the proportion of a given military population that developed noise-induced hearing loss or tinnitus during military service, the amount of hearing loss incurred, or the relative risk of noise-induced hearing loss or tinnitus for a given individual, based on his or her branch of military service, occupational specialty, or service area.
We can, however, look at later numbers, and you'll see the improvements over time which should at least help to suggest how widespread it likely would have been. This chart is from McIlwain, Gates, and Ciliax, and shows levels of 'acceptable hearing' for Army personnel based on service time, and grouped by time in service. You'll notice that the 1974 group, for those hitting 20 years or so, it at about 50 percent outside of 'acceptable'. These men would have joined after World War II, but possibly seen action in Korea, if not at least Vietnam. The 1984 and 1994 groups obviously had much more improvement, and this wouldn't just be a reflection of the fact that those in 1984 saw comparatively little action, as it shows for the service groupings who would have been in the military for Vietnam as well.
Now, while it is perhaps a given that better understanding of these things would mean better treatment and prevention, I'll quickly go over how those improvements developed. Prevention measures were developed in small steps, and it wouldn't be until after World War II that real efforts to minimize hearing loss - as opposed to treating after the fact or providing disability compensation - took off, almost literally, in fact, as the Air Force would take the lead, due to the rise of noise with jet engines entering service. AFR 160-3, titled "Precautionary Measures Against Noise Hazards", would be the first proper regulation within the US military intended to help prevent hearing loss, but even then, it would take several years for real progress. It would be in 1956 that a proper program for hearing conservation was implemented in the Air Force, and more broadly for the military, the Korean War would be passed by when the Armed Services Committee on Hearing and Bioacoustics released their December 1953 report titled "Biological Effects of Noise Exploratory Study", or BENOX for short. The Army's Technical Medical Bulletin 251, titled "Noise and Conservation of Hearing" would be forthcoming three years later, and still was only a guide, not an official regulation. Although the Army would continue to study the problem and make recommendations, they wouldn't pass their own regulation until 1983, with "40-501, Hearing Conservation Program" - over a decade after OSHA!
Works Cited:
Gross, Miriam. "What Do We Hear?" Hygeia Vol 20, No. 7. 1942.
Humes, Larry, Joellenbeck, Lois M., and Durch, Jane. "Noise and Military Service: Implications for Hearing Loss and Tinnitus." Washington, US: National Academies Press, 2005
McIlwain DS, Gates K, Ciliax D. "Heritage of Army Audiology and the Road Ahead: The Army Hearing Program". American Journal of Public Health. 2008;98(12):2167-2172.
Nixon, Charles W. "A Glimpse of History: The Origin of Hearing Conservation Was in the Military?" United States Air Force Research Laboratory, 1998
Wilder, Amos. Armageddon Revisited: A World War I Journal. Yale University Press, 1994.
ETA: Some quick expansion, updated link.
Two different questions are asked: did soldiers get hearing loss? and how did soldiers communicate under loud conditions in the past (did they whisper?).
There were many types of hearing loss. And many types of communication strategies for communicating in noise.
During WWI, 2.4% of British soldiers were disabled from combat because of noise. The nature of hearing loss was less understood and it was often taken as only temporary or as malingering (faked hearing loss). Even through most of WWII the only method to decrease noise exposure was to use cotton or to occlude the external ear canal with fingers. Neither of these methods are effective. Even now, protection is often removed to allow stealthy communication.
Communicating stealthily and effectively with hearing loss in noisy environments is not easy. This was accomplished through a variety of means including hand signals/ sign language, radio signals and words that can be heard well (spondees like alpha, bravo, charlie). Most soldiers did not have meaningful hearing loss in the lower registries and could whisper.
There are two types of hearing loss: tts and pts. temporary threshold shifts are with acute noise exposure and largely return to normal within a few days. less loud noise is needed to cause a tts than a permanent threshold shift. permanent threshold shifts are permanent and irreversible happening with prolonged noise exposure.
These changes can be subtle to profound. They can be measured with conventional audiometry (raise your hand when you hear a sound), or OAE (otoacoustic emissions where sounds are presented to the ear and cellular mechanisms in the ear send a sound back out of the ear, in some cases a different sound than what was presented), or ABR (auditory brainstem response where brain waves are measured). More recently, loss of intelligibility without loss of hearing in quiet has become more appreciated (hidden hearing loss). In all cases, hearing loss will usually center at 4 kHz and expand from there until it gets into the speech frequencies.
With unprotected rifle training, the left ear is toward the muzzle and is more affected. Artillery, particularly in the navy where the sounds rebound within the ship, had the greatest levels of hearing loss. (Now, it is people that take care of fighter jets.). Explosions, especially near the ear can perforate the ear drum (often with bleeding) and cause other types of mechanical damage. The Boston Marathon bombing was one of the most known single incidents where damage to hearing from a single source was measured.
REFERENCES: This is my specific area of research expertise with current funded research.
Cain, P. A. "Update-noise induced hearing loss and the military environment." Journal of the Royal Army Medical Corps 144.2 (1998): 97.
Durch, Jane S., Lois M. Joellenbeck, and Larry E. Humes, eds. Noise and military service: Implications for hearing loss and tinnitus. National Academies Press, 2006.
Conroy, K., and V. Malik. "Hearing loss in the trenches–a hidden morbidity of World War I." The Journal of Laryngology & Otology 132.11 (2018): 952-955.
Kerr, Madeleine J., et al. "Historical review of efforts to reduce noise‐induced hearing loss in the United States." American Journal of Industrial Medicine 60.6 (2017): 569-577.
Le Prell, C. G., et al. "Increased vitamin plasma levels in Swedish military personnel treated with nutrients prior to automatic weapon training." Noise & health 13.55 (2011): 432
Remenschneider, Aaron K., et al. "Otologic outcomes after blast injury: the Boston Marathon experience." Otology & Neurotology 35.10 (2014): 1825-1834.
Saunders, Gabrielle H., and Susan E. Griest. "Hearing loss in veterans and the need for hearing loss prevention programs." Noise and Health 11.42 (2009): 14.