Ambience

Ambience (sometimes called cave noises) is a set of sounds that occasionally plays when the player is in close proximity to a dark cave. Despite their name, they are not sounds that one would expect to hear inside of a cave, such as falling pebbles or dripping water, but rather supernatural noises frequently likened to ghostly train whistles, bells, distant jet engines, singing, demonic roars, and screeching violins. Occasionaly, the sounds play as lower-pitched versions.

Ambient sounds require that a space at least 3×3×3, entirely less than light level eight (with lower light levels being more likely to play the sound), and with no lighting contribution from the sky, be positioned ten to twenty blocks away from the player in any direction. The space does not need to be sealed off - a sound can even play from a space that the player can actually see - nor does the player need to be underground to hear it. The sounds can also come from player-created spaces, even if that space is a box above the clouds.

Trivia

 * Contrary to popular insistence, ambience is not an indicator of nearby ore, dungeons, or mobs. Following them can still lead the player to such things indirectly, since they emanate from unlit caves. Note that the sounds can come from dungeons, if there's enough space.
 * Ambient sounds were created by C418, who also composed Minecraft's music.
 * Ambient sounds are found in the resources/newsound/ambiance/cave from the .minecraft folder.

Höhlengeräusche Фоновая музыка