Thank you for your input. I did what you said and the movies which support Dolby Digital are played in Dolby Digital 5.1 now.
But when it comes to the TV section (my Set-top-Box is connected to Kodi via the Enigma2-Frontend) Channels and programms which contain Dolby Digital make the display of my A/V-Receiver show "Multi-In". Is this the same as Dolby Digital? I tried to compare and it seems that "Mulri In" is a bit louder than "Dolby Digital" - so maybe "Multi In" is more like Dolby Surround - but I'm not sure about it...
Do you know what "Multi-In" is and how I can use Dolby Digital for the TV section, too if "Multi-in" isn't Dolby Digital?
Thanks again for your help so far and I would be glad to get more input.