When you receive a new email message, Thunderbird plays a sound to alert you. By default, it uses your operating system’s new mail notification, but you can change it to anything you like.
Change the New Mail sound
1. In the menu bar, click the menu and select .Click the menu button 
and choose .
2. Select the tab.
3. Select Use the following sound file.
NOTE: If Use the following sound file is grayed out, ensure the Play a Sound box is checked.
4. Click , then browse to the folder on your computer that contains the .WAV audio file you want to use.
TIP: Sound files in other formats such as .MP3 can easily be converted to .WAV. Use a media player for your OS to convert a file, or a freeware application available on the web.
5. Double-click the sound file, then test it by clicking .
6. Click to save your changes.
