Can I check what you’re doing when trying to start music in the app?

Drag up the Now Playing area from the bottom of the screen (OR tap the name of your system top left on the app Home Screen). This brings up the list of speakers.

Tap the speaker name you want to play music on. This becomes the live speaker. 
Then search for music, and tap the track you want to play. It should then play on the live speaker you selected. 

Is this not happening?

Ronny

Thanks for your advices. The most disturbing thing is that the system comes and goes. Sometimes the system shows all spekers, but no sound (content. missing). Sometimes the system shows that nothing is installed. Sometimes all works fine. Very frustrating

​@Knotas 

Can I suggest the following:

• Force close the Sonos app on your phone/device.
• Turn off your router and all the Sonos speakers at the wall.
• Leave them off for five minutes.
• Turn the router back on and let it fully load up to green lights. 
• Turn each speaker back on, allowing each to fully load up to white light before moving onto the next one. 
• Re-open the Sonos app, and see whether things are now working as they should.

I’ll do that, and give you feedback later on

That helped. The only problem was that I had to start two loudspeakers manually. Thanks a lot