Is the mail app able to be in full screen? Will notifications come if you're in another full screen app?
I found something incredible. Go Preference/Viewing in Mail and change the view to classic. Quit the app, reopen. The sound came back, unbelievable !!! It works.
^^I concur. Mail, Calendar, twitter and Due are working fine for me.
But I don't get any for Mac Apps Updates and I have to manually launch the app to check and it's not listed in NC Settings.