The simple fix for this bug then, is to disable and remove all notification sound options from Thunderbird, on OS X 10.8 and above, and let Notification Center deal with it.
Since OS X 10.8, display of notifications - alert banners/dialogs, badges, and notification sounds - are now the responsibility of the operating system, not the application.
When you go into Thunderbird > Preferences > GeneralĪlerts and default sounds can be stopped by the notification field in system installation. However, I note that you are using MAC OS and I have heard of something similar before.