There's a bit of theories surfacing about the lore of Minecraft. I have chosen two which seem to have good foundation to them. I will try my best to explain them.
Enderman used to be a civilization of humans who retreated to the end and mutated into what they are now. They were a joke meant to be Slenderman, but they may have some backstory to them. This might sound weird, but it may be true. Enderman are one of the neutral monsters. They aren't hostile like illagers, zombies, ect. but not friendly like animals or villagers. They also are the only mob that can pick up blocks and place them. No other mob has that ability except for you. They also speak English, just really distorted. They say things like “Hi” “What's up” and “look for the eye” most likely referring to the eye of ender. Now we get to the structures: Abandoned mineshaft, stronghold, desert temple, jungle temple, ect. All abandoned but mostly likely built by the old civilization. How does this tie into the civilization? Hostile monsters ones who kept attacking them and almost wiped them out (the zombies and skeletons could be remains of them) so the had to create structures and some fit with traps (like the jungle temples dispenser traps and desert temple TNT) to try to keep them safe. The stronghold was most likely a last ditch effort structure they built to try and survived. Things weren't looking great for them, so they then created a portal to the end. (Keep in mind that they must have been pretty advanced to do that, as the only portal you can build is the nether. There is no crafting the end portal, all you can do is find it). They went in, but they didn't know it would be a one way ticket. Now they arrived at the end. Here they discovered the Enderdragon here and most likely attempted to fight it, but with no luck (as it is still there when you arrive). Now here we are at the end cities. The humans must have built them with the weird blocks they found in the end. It is obviously something made by them as they have wooden doors and loot such as swords, horse armor, and other things. However, these items cannot be used by Enderman, but here we get to that. The humans probably.realized they were trapped worth no escape and started eating the chorus fruit, which starts to give them teleporting ability. They evolved to get taller, and due to the end being dark, adapted by changing their eyesight to night vision ( when you spectate enderman, their vision is inverted). They also fight endermites. Endermites are probably silverfish that came to the end with them, as strongholds are infested with them and evolved the same way as Enderman. This would explain why they fight, as it's basically player vs. silverfish. The Enderdragon is the oppressors keeping most of them there. (Fun fact: when the Enderdragon was added, Enderman's eyes changed from green to purple). They also adapted from a need to rely on water, so it ends up hurting them. When you kill the Enderdragon, you are freeing them. The second theory may tie a little bit into this one.
Cave sounds have remained a mystery for quite some time. Music discs 11 and 13 also, as no one has been able to explain. The answer may have been right in our faces. This theory is called the crash theory, which is about a crash in a mineshaft involving the death of two miners, and one to be attacked by an enderman. This theory starts off with miners in a minecart riding on rails. A creeper is there and notices the players on the minecart, so it travels over to their course (This is cave sound 14, and when put into a spectrogram, shows the face of a creeper). It blows up, injuring the miners and you can hear the minecarts tumbling down, falling and pushing others. The miners are on the tracks as a minecart engine can be heard. It then crashes into them, killing the two while one lives. While they become zombies/skeletons, the last one is trying to get out. An Enderman must have traveled to the location to see what the noise was, when the miner makes the mistake of looking into it's eyes. This is where music disc 11 ties into this: it the recording of the miner on the run from the now aggressive Enderman. 13 is most likely the aftermath of it. During 13, you can hear sounds of TNT and water, as well as metal banging, which would be the minecarts.