What is playsounds?: Play sounds is a command that plays the sounds in Minecraft. If you do /playsound [sound file name] [player]then the game will play the sound you typed, this is a list of all those.
There are different versions of some sounds. If it says for example [1-5] it means you can type 1,2,3,4 or 5 after the sound line. That means that there is different types of that specific sound. Beacuse TNT doesnt Always have the same sound everytime it explodes, right? Well, check out the list!
I HAVE A TEXT DOCUMENT DOWNLOAD FOR ALL THE SOUND NAMES HERE, PLEASE USE NOTEPAD++ TO OPEN IT!
To be honest, I must say that this is relatively cheap.
On a *nix box with basic utilities (a POSIX-compatible shell, coreutils, grep, sed; should probably be preinstalled), you can generate such a list with two commands (not including the sections but I guess these are quite easy to do by hand, especially considering that it is sorted alphabetically, and the index can just be replaced by searching through the list, CTRL-F'ing should do that in pretty much every browser around):
~$ cd .minecraft/assets/sound
~/.minecraft/assets/sound$ find | sort -n | grep ogg | sed -E 's_\./(.*)\.ogg$_\1_' | tr / .
-- Generated list is outputted here --
Full example on how this looks (Here's a truly full list of all sounds (including horse sounds, and all numbered files have their own line, in case anyone needs it):