Isn't the main reason for the success of this measure precisely to make any infected people retain droplets (big or small) inside the mask?
I'm not saying it doesn't offer any protection from droplets in the air, but the main motivation was containing droplets.
In this case, Choir practice it's literally a spit fest - you're singing loudly in a tightly packed large group of people. It's like the perfect scenario for spread.
Even if they are in rows, you get them on your head, neck, shoulders, hands, pants, sheets of paper you're holding, even shoes.
Also, if it is indeed suspended in air, then wouldn't the minimum requirement be N95 masks?