The guys who are serenading the new prisoners are English, the first English-speaking prisoners taken in the war. They've been in this camp for years.
They're all officers, and they have all tried to escape from other prisons at least once.
They can't break out of this one, though, because there's nowhere to go: the POW barracks are surrounded by camps of starving Russians. Even if they managed to tunnel out of their own prison, they would just wind up in another one.
The Englishmen are in great physical shape: they've basically been spending the last four years working out and eating Red Cross rations.
Thanks to a bureaucratic error, the English soldiers have been getting extra food and supplies that are keeping them better fed than pretty much anyone else in Europe during the war.