All the time. I am not a good singer at all, but I love it. If I'm riding by myself I'll usually have a song stuck in my head anyway so I'll start singing if the mood hits. Usually it's on long descents or flat sections. If I need some positive reinforcement, I'll change the words of a song and sing about how fast I am or how quickly I am going up a hill.

So far I've not embarrassed myself, but the roads I'm on are pretty quiet without many people around.