Of course it is, I never changed it.
You've checked that the "Play a sound" is ticked in Thunderbird?
I wonder if you realise how aggressive this response reads?
Also you were asked to check if the item is ticked .... not whether you changed it or not.
You did not answer the question that was asked of you .... did you check?
I cannot help with your posted problem, but the apparently aggressive responses just might have something to do with the lack of help available.