I think that, before you decide if there are atheist songs, or not, you have to consider the different things that "atheist"can mean--

"Atheism" actually pre-existed Christianity, Anaxagoras was called an atheist, because he believed the sun was a rock, rather than a divine being, Theodorus was exiled because he did not believe in anything divine, and of course, old Socrates got into a lot of trouble because he didn't believe in the State Gods--More recently, perhaps in the spirit of Theodorus, Spinoza believed that God was the total of everything, and, since there was no separate, "divine being" in his philosophy, he was denounced as an atheist, an odd situation for a person who was also called"The God-Intoxicated Man"--

Mostly, when people talk about atheism, they tend to mean materialism, which is to say the idea that everything can be explained mechanically, or at least in terms of concrete things, and needs related to those things--

I think it is important to remember that atheism is not fundamentally anti-religious or anti-Christian (though Communists, who embraced Engels dialectical materials certainly were anti-christian)--in fact, it is possible to point out that atheism is a fundamental element in Christian thought, if you consider that it refutes superstition, emperor gods, and idolatry--

Quite a lot of material there for songs, if you asked me--