So OS X has some cute little alert sounds it makes while you use your Mac. And if your like me those sounds can really get annoying and old fast, but you still need them. Well you can easily customize them and add your own so heres a short guide on how.
1. Get your sound.
Go and find the sound you would like as an alert. So for example I am a 24 fan boy so I searched and found the CTU (Counter Terrorist Unit) Phone ring
2. put sound into sounds folder.
Once you have your sound navigate to: User/Library/Sounds and move your audio file into this folder.
3. Change sound in system preferences.
Open System Preferences > Sound, and select your new audio:

Ta da! Now your new sound is in OS X. You can even use this sound in other applications like Mail.
