Besides trying with volume automation, I tried automating a gain effect pre-fader with a custom global parameter I called “transition”.
In the attached screenshot I selected an event containing the microphone audio und one event containing sonification audio.
On those two events I set an automation curve for the gain using the same global parameter I called “transition”.
With transition I meant that with seeing both automation curves they overlap(as you can see on the screenshot above) at a certain part of the parameter timeline/range so that both of the events are heard at the same time.