It's a known bug that seems to be related to Fast User Switching (while Mail is running). But you might want to give this a try.
Go to /Applications/Mail.app.
Control-click on Mail.app and "Show Package Contents."
Open the Contents folder in the window that appears.
Open the Resources folder in the window that appears.
Control-click New Mail.aiff and choose "Get Info." In the Info window that appears, check "Open with:" and change the application listed--probably iTunes.app--to Mail.app. Don't choose "Change All ..." (unless for some reason you want all aiff files to open with Mail.app).
Under "Ownership & Permissions" (in the same info window) unlock the padlock, change the owner to you, and lock.
Close the info window.
You can use this for other Mail alert sounds as well.
When you're done, restart.
Edited 1 time(s). Last edit at 06/04/2006 10:40AM by johnc.