I would like to share a solution to having no sound despite having a connection through Spotify Connect and despite showing that the app is actually playing the track - but with no sound as said. So far not a single forum entry showed me any work around to fix this - so maybe you find this helpful: If you hear no sound but Spotify is playing (I had the same problem on both iPhone 8 and Mac El Capitan) switch your receiver to "Online Radio" and you will probably hear the radio playing with sound; go then to Spotify and connect again through the App to Spotify Connect and start playing. Voila, suddenly it worked. This is very strange but I have sound again.
I own a Denon AVR-X2300W and every so often it happens that Spotify won't play. I never found a good solution other than waiting for it to miraculous start working again. Your solution worked directly, so thanks for sharing.