It's not too hard. But Robert Klein, in one of his early-
70s bits, rightly lampooned the range required sing both the low "Oh, say..." and the high "rockets red glare".
The tradition of singing it before US sporting events dates back to the patriotmania of WWII; zillions have now done the honors - some sports teams have regulars, like Chicago's Wayne Messmer, while another tradition has just about anyone sing (the Sales Manager's neighbor's "talented" niece, e.g., or Miss America).