Looking at the reasons that British soldiers were discharged, there were some cases of "deafness" being given as a reason. However, these pale into insignificance when compared to rheumatism (easily the number one reason - all those years of sleeping outdoors in the wet and cold on campaign) and also eye problems. The noise of battle would have been staggering loud to men of that period but such actions were very rare, and practice firing rarer still, so there would have been little sustained impact on the hearing to cause long term problems (though I imagine most soldiers would have been temporarily deaf to some extent after a battle).