I think only sims with certain traits should sing in the shower. I made a grumpy sim and it just feels out of character for him to sing while he is showering :/
I've noticed more of my Sims singing in the shower lately. And one of my gloomy Sims, during a Winterfest gathering at the bluffs, just stood there and sang, which really annoys me. I'm going to have to cheat to take that skill away now. :|