Lots of things! It's impossible to classify what you're likely to hear, although some things are more likely than others (bad language is rare, for example, because players who use it are punished almost immediately by the wizards and witches).
What is often confusing to newcomers is the strange call "icons?" followed some minutes later by a countdown from three to one. This is where a group of players have got together to tackle a particular puzzle which involves their issuing a MEDITATE command simultaneously in certain places about The Land. Once you've got the feel of the game, you'll find out more about it from the other players.
Some players like to keep everyone informed of their activities. You often hear people shouting "hi" (or words to that effect!), and bidding you farewell when they leave (often saying where they have left the useful objects that they had accumulated while playing, eg. keys). These characters will usually supply a ready stream of amusing incidents as they stumble about The Land in their quest for treasure, although not all are friendly...You will occasionally hear appeals for help, when players have been attacked by some of the more vicious creatures. Wait until you have experience of fighting before thinking of joining in, or you might not last too long. Sometimes players shout for help to lure others into a trap, but thankfully not TOO often..!
Some wizards and witches will set quests or quizzes for players, by shouting out questions and awarding points for the correct answers. This can be fun, and you can join in while carrying out your normal exploring/treasure-seeking activities. If a visible wizard or witch shouts, you will be told their name, rather than the 'male/female voice inthe distance'. Likewise, they will be told your name when you shout - anonymity is not guaranteed!
Finally, not everyone likes shouting and you will sometimes see shouts like "QUIET!" and "SHUT UP!!!". These people are trying to catch some sleep to restore stamina, and shouting wakes them up! If they're powerful magic-users, you may find yourself being 'dumbed' if you persist. You have been warned!