That somewhat depends on your period, but I can somewhat address this issue for the Civil War, at least to some extent. There were a number of formations used during this war, depending on numerous factors which aren't really relevant here, but a general formation for combat, known as "in line of battle" (Winfield Scott, Infantry Tactics: School of the Soldier and the Company) placed troops two deep with 13" between them. The rearmost set would have their rifles over the right shoulder of the man in front, and this combined with the length of the gun would place the muzzle ahead of the front man's head, and the percussion cap behind and away from the ear. Thus, the distance from either explosion would be increased, helping to preserve hearing. Functionally the ear was between the second and third band of the gun, if that helps visualize it.

in general a longer "three bander", such as the 1853 Enfield or 1861 Springfield were common guns, but as with all things Civil War the variety of guns used was broad.

Now, whether or not this was due to a knowledge that hearing could be lost from gunfire, or simply the way it worked out due to the formations used, I am not sure. However, this would be one way which hearing could have been preserved, even if by accident.