Note: I could not find any studies that estimate how many heavy metal bands are atheistic, so "most" is nothing more than a personal observation.
Chances are good that if you go to church, you sing. Most churches around the world; be it Anglican, Catholic, Orthodox, or Protestant; have singing as a part of worship. Every Sunday they meet, greet, sing, preach, share personal stories, and some then sing some more. Why?
The first time that I sang was in college in voice class. It was the single most enjoyable and fulfilling experience that I have ever had. I was awful, but there was this intense sense of unity, this sense of belonging that I had never experienced before. There we were, a group of just 20 or so students, and together we all made a work of art for the sake of of making art. It was beautiful. I had never felt so connected to people that I did not know before then, and ever since I stopped going to that college I have not felt that sense of connection to others so intensely. I do not go to church. I have not gone since I was a little kid. Yet, almost every day I am consciously envious of the people who can believe in God because of how beautiful that singing, that sense of community, was.
I believe the reason why so many churches have singing is because of this sense of community. Singing is a readily accessible and simple way to bring people together. Churches that don't sing don't build a sense of unity with singing, and people will go to the closest church that they feel the most belonging in. If churches that don't sing don't have other ways to supplement this sense of unity, then Darwinism happens: Churches that are less able to create a community are less fit to survive.
What if you don't believe in God? What if you're a kid, a teenager, and it's Sunday and your friends are out playing and having fun and going to the arcade or playing football and your parents instead make you go to church? The Sabbath takes your day of rest and turns it into a day of work. Instead of getting to relax you get to be angry. Angry at your parents for keeping you from your friends and for not loving you if they were to ever find out that you do not see the world the same way they do. Angry at the church and the people within it for hating the nonbelievers and gays and anyone who just doesn't belong. Angry at God for being a convenient weapon for this community, that you do not feel a part of, to use against you. And you sing.
You get good at singing, as you sing every Sunday and have every Sunday for as long as you can remember. Your puberty goes by filled with stress, as all puberties do, and yours gets to be filled with an extra dose of anger and alienation. And you sing some more. But what do you actually want to sing about? What emotion do you have that has gone unexpressed that you want people to hear? How do you want to be heard?
And you get mad.