All the time, unless I'll really embarrass myself by doing so.

I play music on my computer at the office (and sing along if my coworker is out to lunch).

I worked in the dining hall kitchen in college. My favorite job was unloading the dish machine because the machine was so loud I could sing all I wanted an nobody could hear me.

(I'm a really lousy singer. I try to be polite and not sing out loud when other people are around because it's really not a good thing.)