weird .I don't use thunderbird [did for awhile a long time ago],just install it ,and that must be a default addon ,only one i have too

Check Tools>Options; Advanced tab; click Config Editor.. button; filter for 'sound' and ensure that mail.biff.play_sound is 'false'.

If so, my guess is that your profile is corrupt. Maybe try backing it up (browse to C:\Documents and Settings\[User Name]\Application Data\Thunderbird\Profiles\ on Windows XP/2000 or C:\users\[User Name]\AppData\Roaming\Thunderbird\Profiles\ on Windows Vista) ; you could simply move the profile (make sure Thunderbird is closed) and on the next launch when a profile is not found a new one is created and you can try a fresh start.

Turn off your speakers.

I finally found the solution. In Thunderbird, select the menu Tools --> Spamato --> Configure Spamato On http:://localhost. Select "Sound" from the menu in the left-hand pane, then change the radio button to "No Sound".