Thank, Marius.
That is exactly the problem – the room is simply not big enough to “hide” the speaker behind the screen – I would have put it in front of and below.
If I put it directly under the screen and the projector below the speaker, I would loose too much space as the distance between projector and wall(screen) would be too big and I would have to put it nearly in the middle of my room so that the speaker would not disturb in the projection.