I didn't even recognize it as "happy birthday" at first. It's quite solemn and even regal in a way. Makes me think of a desolate, snow-covered kingdom with an ancient castle looming oppressively overhead. I understand this was a quick thing, but I'm not sure I like the instruments too much (or maybe it's just the way you balanced/mastered them, idk). Still, interesting mood here. Congrats on finding a way of making a song that literally has the word "happy" in its title...well...extremely depressing! :O