Sound
NetHack has no sound in the sense of audio-output from speakers, but your character in the game will hear many sounds, such as:
You hear water falling on coins. You hear the splashing of a naiad.
Most sounds have a definite meaning. All of the above tell you that there is a fountain on the level. The next reveal that there is a vault:
You hear the footsteps of a guard on patrol. You hear someone searching.
Other sounds can indicate one of many events, such as:
You hear a chugging sound.
This means a monster quaffed a potion, but you don't know which one.
You hear crashing rock.
There's a dwarf or some monster somewhere on the level that is creating tunnels through walls.
You can use this wiki to look up what a sound means by typing its message into the search box. You should be redirected to an article about the thing that caused the sound.