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. Click Thunderbird app menu > .
2. Select the tab.
3. Select Use the following sound file.
4. Click , then browse to the folder on your computer that contains the .WAV audio file you want to use.
5. Double-click the sound file, then test it by clicking .
6. Click to save your changes.