My friend's dog does the same thing to the same song! She even sings songs like 'you are my sunshine' as his name is Sunny and he's fine, but boy when you start singing 'happy birthday' block your ears! We always wondered about this, and we were thinking, since dogs have a better sense of smell than us humans, maybe they associate the song not with people cheering and clapping, but with the fact that he can smell the lit candles and perhaps he senses danger. Dogs are pretty smart, he might have just learned to associate the song with the danger of fire. And until dogs can learn to communicate with us, I guess we'll never know ;)