I sing quietly to myself, as my singing ability is what one my describe as awful. However, as a born and raised Baltimore boy I feel it is my sworn duty to loudly shout "O" when the song gets to "Oh say does that star spangled banner yet waive" at any sporting event. I'm not sure when this tradition started, but we've been doing it at Orioles games (and any other sporting event) for as long as I can remember.
On a side note, of you ever find yourself in Baltimore, take the water taxi to Fort McHenry. You won't regret it.