Hi, JI070577.
Sorry, I don’t use Thunderbird, so I’ll ask if these settings are already done on your machine: https://support.mozilla.org/en-US/kb/how-customize-new-mail-sound ?
If that still doesn’t help, there is a suggestion here https://forums.linuxmint.com/viewtopic.php?t=218478 to install “mail-notification” (I confirmed that this is available on Synaptic).
Let us know if either of these help.