Create custom alert sounds for your Mac

There are a few easy ways to capture new sounds.

• The first is to use Snow Leopard's QuickTime Player to recording new audio right at your desk.QuickTime Player > File > New Audio Recording. On the right side of the Audio Recording Window you will see a small arrow button pointing down. Clicking this will reveal a pop-up menu that lets you choose a microphone—either the one built into your Mac (Built-in Digital Input: Digital In)
• Another, more portable way to capture new sounds is to use the Voice Memos app on your iPhone or iPod touch. Once you’ve captured a sound, tap the list button in the bottom right-hand side of the screen. Now choose the sound you just recorded, tap Share, and send it to yourself via email.

Adding sounds to your system

• Finder > Go > Home > library > Sounds : Simply drag and drop the audio files into this folder; once there, they’ll be available to all applications that use alert sounds.

Assigning sounds

• System Preferences > Sound: you’ll see a list of alerts that should now include your new custom sounds; simply click the one you want to select it.

source: http://www.macworld.com