Silence annoying sounds
If you like having audio feedback when your Mac has completed a task—such as moving something into the Trash—open the Sound preference pane and enable the Play User Interface Sound Effects option. For the most part, these sounds are unobtrusive. But you may find that one really grates on your nerves. Although the Sound pane doesn’t let you turn off specific sounds—it’s all or nothing—there is a workaround.
In the Finder, navigate to /System/Library/Components. Control-click on CoreAudio.component and choose Show Package Contents from the contextual menu. In the resulting Finder window, navigate to /Contents/Resources/System Sounds. Within this folder are subfolders containing AIFF files of all the sounds your system makes, each clearly labeled.
Before deleting one of these files, back it up by dragging the sound file to another folder. Then highlight the sound file, drag it to the Trash, and provide your administrative password.
You may need to restart the Dock, the Finder, or the system itself to have the changes take effect. You can easily restart the Dock and Finder from Activity Monitor (in /Applications/Utilities).
If you’re truly adventurous, you can replace the existing sounds with your own audio. Simply create an AIFF file, give it the same name as the file you’re removing, and then drag your replacements into the proper folder. But make sure you have a backup of any sounds you’re replacing—you may tire of hearing “D’oh!” every time you empty the Trash. —RG
http://www.macworld.com/article/1046730/tigertips1.html