Just a quick update: I managed to implement the millis() function correctly into my code. Before the sound was only triggered, when both pedestrian and cyclist are detected simultaniously. Now it also triggers, when a bike is detected and a pedestrian has been detected recently (within the last four seconds).
So I’ve got the main bits up and running. Now its time to exchange the buttons for the sensors again. Then start prototyping, to adjust the sensors sensibilities, and go do some testing.