@Mushroom
I doubt most have ever had a chance to actually hear this happen.
True. I doubt you'll find many people in a situation where the rounds passing by are very different to the rest. However, I once made the mistake of accepting the request to help out at a rural rifle range.
Like most rural Australian rifle ranges this one was built shortly after WW1 with the intent the locals would keep up their shooting skills in case of further need of good shooters. The ranges were built on the cheap and were simply flat stretches with a dirt backstop put up just outside of town. In most cases they took some council owned land then make it perfectly flat by using a bulldozer to push the dirty to the end of the range to make the back stop. Then they paced off firing lines from where the targets were set just before the back stop. Sometimes they dug a trench for range staff just in front of the targets, sometime the trench was just behind the targets, and sometimes the staff were behind the backstop.
On this range the trench was in front of the targets with targets about a metre or so behind the trench. Depending on the type of shoot the range staff either marked each shot or each set of shots. In either case, they waited until the range officer at the firing line gave the all clear on the radio before standing up to leave the trench, walk to the targets, and mark the holes with red dots. Normally all the rifles being used on the firing line were of the same calibre group - under .300, .300 to under .500, .500 and up.
Anyway, on the day I was helping in the trench they had a small group of shooters so they had them all shoot at once to get it all done early as the nearby pub didn't open the bar until after they were finished on range days. Anyway, as the firing line was setting up we walked down to the trench, and the regular local showing this poor visitor the ropes set me up in the middle of the line. As he did he said, "You'll get a treat today. Listen to the rounds going by." I had no idea of what he was talking about until later.
For some reason I never got told the shooters fired each round in order. Thus position one fired then position two as soon as one fired and so on. The result was you if you were at the line it was a series of bangs as the rifles went off, but at the trench end three hundred metres down range you heard a couple of round go by before the mode distant bangs were heard.
When the first set was fired I could hear a slight difference in the sound of the cracks as they went by, but I could never clearly state the difference in the sound, only that they were different. As we were marking the shots for the first set the local told me I was sitting at the spot where I had the .22 firing on one side and the .303s on the other side. A sat the other way round for the next set of shots, just to see if it was my ears, but there was the same difference. the best I can say is it seemed the .22 had a slightly higher pitch, but I'm not sure what it was. The local told me he sometimes heard a difference in the sound when another local was firing his WW1 .50 cal anti-tank rifle while the rest were firing their .303s. He also said he could tell a mild difference between a pointed round and a flat faced round - he was right on the others, so I take his word on that.
Later in the day they closed up to let the few guys who had legal pistols have their go, and they didn't have the crack of the rifles. Often you couldn't hear the round for the sound of the gun itself - but that was at twenty metres.
This was way back before the crazies got the government to disarm the people to leave them at the mercy of the armed criminals.