Instructions
Open your "MapleStory" directory. The directory could be located anywhere on your computer, depending on where you installed the game's files; but the game's default directory is located in the "Program Files" folder. By double-clicking on the "Sound" folder, you can locate the "Sound.wz v75" file.
Drag a new sound file named "Sound.wz V75" into the folder. A message will appear asking you if you would like overwrite the file currently in the folder.
Click "Replace" from the window. The old file will be erased and your new sound file will be placed inside of the folder. The next time you start the game, your custom sound effect will be played instead of the game's default sound effect.