It's all in the config.php file. There's a block of code that defines what events play which sound file. All you do, is upload the new sounds you want to play into the sounds directory, and change the code in config.php to match the names of the files you just uploaded.
I believe to turn the bell on, you need to modify the layout files for users, admins, and mods. The layout files for each usergroup is found in the inc/layouts/ directory.