I sing, hum or whistle frequently throughout the day; my brothers do, too (though our parents don't). I absolutely belt in the shower. I've almost always got a song going through my head. I even wake up remembering not only the tail end of a dream but some background music to it—sometimes it's music I've spontaneously made up!

I've been increasingly inclined recently not to stop humming or singing as I pass someone in public, but rather just to attenuate the volume. Since they've probably heard me as they approach, I rationalize that not stopping makes us both feel slightly less like strangers/intruders.

Though I must admit that I'd rather not hear most other people sing on the streets. The average taste in music is dreadful--we're inflicted with it enough--and my dog can carry a tune better than some folks I've heard.