Well, here's one on the topic.
But I object. There are many songs that do not have any affiliation with anything religious, but they are perhaps better designated agnostic songs? Then, there are also many songs written by atheists, but that doesn't make them especially atheist songs, I suppose. As when a religious person writes a song nothing to do with religion (does that happen?). But then, songs about non-belief? How about John Lennon?
Update 11/10: Via the comments, more atheist songs:
4 hours ago in Variety of Life