There are many answers on the Internet, the most credible one is "Green Devil".

This is the military song of the German airborne troops. Some say it is the song of the Viking division, but judging from the lyrics, it is the song of the paratroopers.

The following is the inference of netizens:

“Of course, I don’t know why the soldiers of the SS Youth Army sing the military song of the paratroopers. Maybe the director also has insufficient research. Right.

Does it refer to the group of soldiers the protagonist met while standing guard under the tree? If it is those people, it is probably singing this song (note the line "Wo wir sind da ist") immer vorne, und der teufel der lacht noch dazu: Ha ha ha ha ha! It’s exactly the same here) except that the movie has background music when singing, and the background music is another song. The two songs are stacked together, so you can listen to it. It doesn’t look like it.”

I’ve also watched “Fury,” but it’s definitely not the “Mass” that many people are talking about. It should be "The Green Devil".