Smiling changes your whole sound. It relaxes your face and changes the whole shape of your vocal tract. It helps pull your voice up to the front of your mouth. That makes your voice brighter and more distinct so your sound is clearer.
Smiling also has a very positive effect on your mood and your outlook. It’s hard to feel grumpy when you’ve got a smile on your face — and that means you’ll feel happier when you sing.
Most importantly, a smile is contagious. Your audience will engage much more with a happy, smiling choir who are obviously enjoying themselves.
And yes, smiling even helps sad or serious songs sound better. The smile may not fit the mood of the song but it still makes you sound better.