I listen to music all day at work with my headset on, either my iPod or Last FM. Occasionally I will sing outloud and my co-workers get serenaded (not intentionally, I just get lost in the music and forget).
Here is one song you do NOT want to sing outloud at work:
Star Star by The Rollings Stones
I'm sure there are many others. Luckily I caught myself right as it got to the chorus.