It seems that the description I’ve given in my first post was not exactly enough. After booting the pi into the desktop audio, video and a usb - camera are fine. I can play files with audio and video.
Inside jitsi there is no audio, no video and no camera. In preferences there is no possibility to choose a audio device except none or silence or a camera except no camera.
I start jitsi from source with " ant run " after an initial " ant rebuild "