It's bad enough that a billion iOS devices all start chiming at me whenever I receive a notification. The OS X notification center plays that awful sound -- Basso, whenever my reminders trigger.
Not sure what that sounds like? Open System Preferences > Sound > Sound Effects and click on it to hear. I hate it.
So I decided to override it. These steps aren't for everyone. You'll need to feel comfortable at the command line for one thing. So caveat hackteur.
Go to ~/Library/Sounds. This is not the same as the system /Library/Sounds folder. It is in your home folder.
Add your (short) sound and name it Basso.aiff. Use an AIFF format file, don't just rename something else. I use QuickTime Pro, but there are lots of other AIFF-friendly apps out ther