I didn't want to open a new thread, but I noticed one odd thing about Thunderbird:
It seems there is a bug when you have more than one imap account. Ever since I added a second account, the notification feature ceased to work reliably, sometimes I got a notification, sometimes I did not. Since I am a freelancer receiving all my work per email, that is a problem as I need to know immediately when I get new mails.
At first I did not think much about it, I thought it had to do with another email program that I had installed, i.e. that they kind of messed with each other's notification feature in the trail. But even after I uninstalled the other client, the problem did not go away.
Now I deleted the second imap account that I had configured a few weeks ago, restarted, and got a weird notification saying I had 12 new messages (there was only one, though). So I opened all mails in the inbox, restarted, still the same message. So I deleted all mails in all folders, now everything seems to work again, including both audio and symbol notification
But I can't use my second account anymore, luckily that is not big problem. But some people need to use more than one account...