Are you using a custom notification sound? Try a different one and see if that works.

A few versions ago, my custom sound stopped working, but only in Thunderbird - Windows would play it with no problem. The sound was one I made many years ago and had been working all along. It was a slightly unusual format - mono ADPCM. I remade it as a stereo PCM format file, and it worked again. It seemed that Thunderbird changed something that made it not able to use that format audio file. I never saw a confirmation of that, though.

This is all a long way of saying, try another file.

You might have to also check that notification sounds are enabled for TB in Windows Settings.