Sound World is a world in Mario's Early Years! Fun with Letters. It is one of the seven islands in the game and is represented by an island with a book containing a dinosaur on one page and the letter D on another.
This world takes place within a library. Like all the levels in the game, Mario goes through pipes to go to the next level. Each level is generally the same, with the third level being the reception desk.
For this location, the player learns how to identify words that start with various letter sounds. There are three words, but only one of them starts with the letter sound indicated on-screen. The player must click this word to continue. There are three levels and at the end of each correct answer, a random character pops up to congratulate the player.