Missing Gmail sound notification in Thunderbird: get it back, add special alert to any account
Thunderbird is able to notify you by playing a sound whenever new mail arrives for any of your accounts, even a gmail pop3 account.
However sometimes gmail sound notification may not work, although all normal pop3 accounts still announce new mail with a sound.
To solve this problem, just empty your gmail inbox (select all messages and delete or move them). It seems that after a while, when a certain number of messages is gathered in the inbox of gmail, sound notification faces a problem.
Another solution would be to install this Thunderbird addon I have here, called Mailbox Alert, that will put a context-menu option in all Thunderbird folders, letting you activate sound notification for any of them. Unzip the archive anywhere you like, and from inside Thunderbird Addons install the .xpi file