This seems to be a common issue for Linux users. My computers are generally muted and so I don't know for sure if it affects me too. I use LMDE/Mate at home.

But, in answer to a related question, I installed ToneQuilla and hey presto, I get sounds. Sometimes even when I don't want them. :-/

https://addons.mozilla.org/en-us/thunderbird/addon/tonequilla/

I tried that with no success. I also tried Sound Plus with no results.

I resolved the issue by installing the version from the net.

…as opposed to the version provided by your distro's repositories?