Voice acting is a feature available in some areas of the game, although Jagex has stated they plan on adding more to the game. Most often speech is used alongside text in the chatbox, such as when talking to an NPC, or the text that an NPC 'says' above their head. The volume of it can be adjusted from the Audio Options screen. It was first introduced in-game on 21 April 2010 in the update Slayer Challenges, when voices where added to the (former) tutorial, Unstable Foundations. You could hear Sir Vant, a goblin and several other NPCs talk, along with the player.