This has been asked before, and in several forums, but still NO answer.
There was a British prisoner of war movie, the oft used theme of a theatrical put on by the prisoners. They all escaped down the tunnel by employing use of an applause (clapping) device. The Germans were fooled into thinking the crowd was still clapping before they entered to find everyone... GONE.

I have every conceivable movie about WW2, and many prison camp movies, but NONE has the vital APPLAUSE machine ploy. I'm not crazy. It was shown maybe on TV in the 70's or before. B&W as I recall.