Hi Indeed there were at least two add-ons for that in the past and I believe none of them is available. There is another option (not working on all OS), but this would require you to program your own notifications using the add-on here: https://addons.thunderbird.net/en-US/thunderbird/addon/scriptable-notifications/?src=ss.

Some examples are available here

There is nothing out of the box that works at the moment.

Have a nice day!

Install FiltaQuilla to add Play Sound or Launch File to filter actions, then create filters for each contact, e.g. From + is + <contact email>, Action = Play Sound + <contact sound>.