In addition to sounds corresponding to animals and monsters (collectively referred to as mobs) the game also plays music and the occasional random 'teaser' sound clips or ambiance. It is simply meant to be background in the game to put you off your feet.
They occur most often in caves, but the game doesn't actually know if you are in a cave or not. Rather, the game uses light levels to determine when to play some of those sounds. The area checked isn't always immediately in front of you, either. Depending on your flat world, the game doesn't have actual caves, but may be detecting a void. The game looks for spaces with light levels below 8 regardless of where you are.
If what you are hearing is not a mob sound, it's probably safe to ignore it.
Here is a clip of some of the sounds to ignore:
If you are playing on a multiplayer server or single player server with another person, if anybody summons a wither then everybody will hear it as local sound. The same is true for killing the enderdragon.
If you are in the Nether, as a little bonus, ghasts can be heard as nearby/local even if there are none within any nearby chunks. The game only checks that a ghast has spawned somewhere in the Nether (could even be hundreds of chunks away) and plays the sound for anybody in the Nether.
And seriously, there is no, have never been and never will be a herobrine.
If I were to take a guess at the problem it may be because you have gamerule /gamerule doTileDrops false set to false. You can check this by typing /gamerule doTileDrops and hitting enter, this will tell you if it's set to true or false. If it says false simply type (true) after the previous command (This is assuming you are using command/cheats)
Best Answer
Here's Seth's original video on the subject:
For some reason, when there's a bottom half of a door, with no door part above it, it doesn't render. Thus, with a checker pattern of bottom halves of iron doors (inserted using MCEdit), you end up with an invisible wall.
The reason that they don't break because random block updates aren't a thing. Random world ticks, such as the growth of wheat, trees, etc are a thing, but the game never randomly updates blocks. If a block update does occur next to the door half (block is placed or removed, redstone update, etc), then it will break. But never randomly (except maybe by Enderman intervention).