Your boss is a bastard!!!

An iron might help him smooth out his panties.

If I were your boss I'd be happy for you, I mean, my employees creating a positive image for your business, that's so nice!!! Your boss needs to go to the emergency room, I think that pole is too far up his ass...

OP doesn't mention what type of work she does. Quite aside from the fact that 'Happy Birthday' IS guarded strictly under copyright law (hence why restaurants tend to make up their own birthday songs rather than fork out the costs everytime), it might very well have been a totally inappropriate environment to start singing in. If OP works at a telephone based business, then it might have been OK, but if they work somewhere where people expect a level of discretion, such as a bank or benefits office, then there are better ways to wish someone a happy birthday and maintain rapport than causing a public distraction. Of course OP wasn't attempting to be malicious, but without knowing their line of work we can't judge whether her actions were appropriate to the establishment.

14-music becomes public domain after 75 years.

your boss obviously doesn't understand the concept that things like this make customers happy, and happy customers come back and spend more money.

Where do you work?

They seemed to really have rustled his jimmies.

That was super nice of you and your coworkers. I hope it made her smile. Your boss sounds like a grade A prick-headed jack-ass. F him for being a kiljoy.

You can sue him for emotional distress.