I forgot to specify in the title, but this applies to since gunpowder weapons were employed en masse to the 19th century.
It seems that the first rank would have it especially bad if they were kneeling while the second rank fired, as their comrades muskets would be very close to their ears. It seems that even after regular drilling the soldiers would have no hearing left, not to mention battles themselves. Surely this would have affected their ability to hear their officer's commands?
This is one reason why muskets have very long barrels. I can tell you from experience that, firing in a line two ranks deep, the muzzles of the rear rank will be at least 1 1/2 to 2 feet in front of the front rank's heads. It's disconcerting at first, but muskets aren't terribly loud as firearms go. Cannon are an entirely different story. Whereas a musket may use 60-90 grains of black powder (a grain being 1/7000th of a pound), depending on model, a 12-pound cannon uses 17,500 grains, or 2.5 pounds of powder - between 194 and 291 times more. When a cannon goes off a hundred yards behind you, you can literally feel the shock wave move over you.
Modern (post 1900) rifles, which use smokeless powder to propel projectiles at supersonic velocities, are literally deafening, in that every time you fire one without protection you lose some hearing. If a musket is a bang, an M1 rifle is a roar, and an M16 is a scream. Yet soldiers to this day rarely if ever wear earplugs in the field, because it is more important to be able to effectively communicate (never mind hear the enemy) in the short term than to worry about longer term issues. Not coincidentally, there are a very great deal of WWII, Korea, and Vietnam veterans (avoiding the 20-year rule here) with badly impaired hearing.