The sound of our 155mm Howitzers firing was a reassuring sound that we were either helping someone out of a jam or firing counter battery for ourselves. Depending on the charge and where you were located this friendly sound could be just loud and safe or it could be extra loud with a concussion that would impair your hearing for a long time afterwards even years and would never go away. Impaired hearing and tinnitus were and are a hazard to the artillery man that carried on into his old age. Ear plugs were unheard of back then and few thought of ear protection.
Another pleasant sound was that of the jungle especially at night. A cacophany of birds, insects and animals was somehow reassuring with the exception of one unpleasant creature who screeched out a loud "FAOUK KEW". At least you knew where you stood with him and what he thought of you.
The sound of a chopper could be discerned from a great distance. Heads would turn towards the direction and try to pick out the speck in the distance. You could tell whether it was a huey, chinook, a cayuse or sky crane without even seeing it. They sound of a chopper was a friendly sound because it could mean resupply of food, water or ammunition and even the all important mail. It could mean a medevac to take your wounded in to the evac hospital and safety. It also meant you were moving out and towards another fire base which could be good or bad depending on the situation. It could also mean your ride was coming for a R & R break or even a chopper on the first leg home.
Incoming rounds were a whole different thing. They could be the chatter of small arms fire shooting at an inbound or out bound chopper, or in the wire on a probe or ground attack.
If you were sharp enough you could follow the track of a 122mm rocket or 75mm recoilless rifle round in. If it was a dot you were probably in trouble. If it looked a little oblong you knew it wasn't going to hit you but you still sought out cover because of the shrapnel. A mortar round was impossible to spot mainly because if you were lucky enough to hear the thump when it left the tube you ducked for cover. Then you waited for the explosion.
It's difficult to describe an explosion and depends on what they were, how close they came and what they hit. Unluckily there were the WIA and KIA that probably didn't hear a thing.
There was the loud rolling boom of an exploding 122mm rocket. The whump of a mortar round and the sharp bang of a recoilless rifle round. Hitting dirt or sandbags muffled the sound somewhat and mud definitely quieted the explosion. Sometimes there would be the crack of a wooden beam. You might hear an extra sound if a round hit SSP, PSP or the side of a conex like the FDC bunker. Then you might hear a clang or, if it penetrated, a louder metallic sound followed by zipping shrapnel. There was always the shrapnel to worry about whatever the munitions along with the flying debris. When a recoilless rifle round hit a gun on say the right or left shields which happened on several occasions, it made an unmistakable loud and piercing ‘twang' followed by the zip of the shrapnel and possibly pieces of the gun itself. I've seen a whole gun crew get knocked to the ground either stunned or WIA when this happened.
One sound wasn't really a sound at all but the lack of it. When the lanyard is pulled and nothing happens you have a misfire, or hang fire. Tensions build while you wait because you can't allow the breech to open until a safe amount of time has passed to allow a possible cook-off to finish. One of three things can happen. The gun suddenly fires after a couple of seconds, the primer misfired and after waiting you replace it and fire again. The last is the most serious where a small spark is still burning and any air allowed into the breech by opening it can allow an explosive flash fire and a possible exploding breech. Sometimes the round is even pushed up the barrel a short ways when this happens and if everything is done well, when it is all over the round has to be pushed out of the end of the tube. Once a gunner leaned over to see if the primer was dimpled after a hang fire and the gun went off. It pancaked his head and he only lived a very short time. No sound when the lanyard was pulled was bad news.
I sometimes watch the explosions in the movies where there is a huge billowing ball of fire with the explosion. I never saw that. I only saw maybe a quick flash followed by a dirty cloud of dust, sand, dirt and debris, but always the dirty cloud of dust, sand, dirt and debris.
The screams or the wounded and the shouts of “MEDIC” also rang out during and after an attack. At times there would just be a quiet hush after an attack while everyone figured out what was hit and how lucky they were. I was once in a bunker where a round from a 75mm recoilless rifle went off about a foot from my head. I vaguely heard a muffled explosion followed by absolute silence because the dirt wall, sandbags, PSP and engineers stakes buried me. That close and I didn't even get a scratch. Lucky, I guess . . . .