Brandy Norwood -- This Video Is Sooo Embarrassing ... Crickets for Subway Singing

Brandy Norwood's on Broadway in "Chicago" ... but she might want to stay of the NYC subways -- after she got humiliated while singing her heart out to a bunch of commuters.

Brandy went incognito -- wearing a hoodie and shades -- and belted out a few tunes, hoping to impress subway riders. No dice.

You've gotta see the Grammy winner fall flat. Best moment ... one passenger giving her an atta girl, and telling her to keep working at it.

Check out the vid. Swing and a miss.