Correct, after a period of 13mins if I recall correctly of not playing any audio, the Ray will go into idle mode, which uses 2.92 Watts.

Edit: found elsewhere on the forum.

All Sonos components take 3 minutes to go into idle mode, unless part of a Sonos home theater surround setup. The 3 minutes until idle can be initiated on a surround setup when audio is paused. If pause or mute is not initiated, idle power mode takes 13 minutes for all devices part of a surround setup after the audio signal ceases from the TV.

Thank you!

Actually I want to use the Ray as my bedroom radio (no place for a receiver and classical speakers) so the additional question is if the device with Sonos app has to be ON during radio streaming?

No, when playing radio on the Ray you can turn off your device with the Sonos app and the radio will continue playing on the Ray.

View the Sonos app as a remote control. Take it away and you have to use other means to control the Ray but the Ray still functions.

Thank you very much Mr.T !

I want to use the IKEA Symfonisk / Dirigera to select and stop the radio streaming and of course to control the volume.

Till now seems that everything could work together, eventually some pitfall possible? 

I have the IKEA Symfonisk remote and Dirigera hub. You can programme 2 radio stations as favourites per remote and they can stop/start and control volume. You would first need to add the radio station(s) as favourites in the Sonos app. I have had no issue using the ikea remotes, they work very well in my experience.