The Notification Center is one of the highlights of Mountain Lion, and we’ve written about a few handy Notification Center tips to help you make the best of it, including one on changing the Notification Center background. It is still a relatively new feature of the OS, and some might go so far as to call it rough around the edges for the simple reason that customizing it is difficult. Even something as basic as changing the sound alert for the Notification Center isn’t supported, at least not in the stock System Preferences. It is still possible to do so, and we’ve got the instructions for you.
We’ll go about this the simple way as well as the ninja way (via Terminal command). The default sound is an AIFF file named Basso, and can be found in ~/Library/Sounds. Whichever sound file you choose to replace it with, must also be in this same AIFF format, and not longer than two seconds. Before you attempt to replace the file, it’ll be a good idea to back up the original one. Next, delete it from ~/Library/Sounds, paste the file you want to use in its place in the same location, and rename it to Basso.AIFF. You will need to authorize the change by entering your password when promoted.
Next, you need to completely quit and restart Notification Center for the changes to take effect. Here, you have the option to go with the Activity Monitor method, or use the Terminal.
Using Activity Monitor
Open the Activity Monitor from Applications > Utilities > Activity Monitor. Make sure you are viewing all processes. In the search bar, type Notification Center, select the process that shows up in the results, and hit the Quit Process button.
To quit Notification Center, run the following command:
Notification Center should now quit.
Normally, the Notification Center should restart itself regardless of the method you used to quit it. If it doesn’t, head over to System/Library/CoreServices/ and run Notification Center.app from there to start it again.
This little tweak shouldn’t have any lasting or irreversible effects. You can always restore the original sound by moving the backed up file back to its original location. Also, make sure the file you use are replacing it with is in the right format and you aren’t just renaming the extension of an audio file in another format to AIFF. While we’ve found little tweaks like this to work around the lack of customization in Notification Center, OS X should really offer the option to do all of this from the Notification Center’s preference pane itself.