IPhone 6s, No notification sound when receiving a new email? All other notifications working well.

I am having the exact same issue suddenly! Not sure how long it's been going on, maybe since December, but when I am in stand-by mode (the screen dark) I get no audio notifications of either text messages or phone calls. If the screen is lit, I do get the sounds. I am not on mute; and I checked the volume, and all is fine.

Go to Settings > Notifications > Mail and make sure sounds are turned on

you're in nightmode?