Singing is only one use of your voice. Take voice lessons to learn how to speak to people in a comfortable voice for you and an entrusting voice to the people you speak to, and that entrusting feeling you want to create varies with every single person you speak to. Singing “Happy Birthday” or the national anthem is nothing particularly important for yourself or others. You can sing both out of tune and key. It will not be even noticed since you sing these in a big mass of singers.