I guess "atheist song" is kind of a peculiar concept, like the "conservative song," on that other thread. There is no feeling in atheism per se; you really can't make a good song about it. The feeling is in the hostility toward human institutions religions may be associated with. And there are lots of blasphemous song, blasphemy being part of the contrarian nature of Man.

I think "The Preacher and the Slave" is the closest thing to an atheist song I've ever heard. Maybe there could be a song called "Unsurprising Grace" or "Completely boring and commonplace Grace"?

Another thought - Perry Como used to sing

"Every time I hear a newborn baby cry
Or touch a leaf, or see the sky,
Then I know why - I believe!

An atheist could easily make up a song structured like that, pointing out all the yucky phenomena that cast doubt on the idea of a just & omnipotent God.