The same for me.
Samsung Galaxy S7
Android Version 8.0.0
Spotify Version 8.5.49.973
Car Radio Kenwood
Using the Car Radio "Spotify-Mode" it usually took about 5-10 Seconds to autoconnect and start the music. The title is shown on the display and music is played.
Some time ago it suddenly happened that the title was shown but no sound. I needed to press "Pause" once to pause and again to start on the radio and it worked.
Since some days it connects, shows the title but no sound at all. I can press "Pause" again and again - no change. When checking the mobile it shows that the app is running and the title is played simultanious to the radio (means as soon as I press "Pause" on the radio or app the title stops on both devices). So the connection itself is fine.
After some clicking and closing and starting the app sometimes the sound is back. I've already done a hardreset on the phone but no change.
This really is bothering me! At the moment it is not possible to listen to music using the app connected to the car radio.
Please help!
Edit two days later:
For me it seems to be connected to the "Offline Mode". In the car I am usually listening to audio books and I do not want to search for the last stop every time I want to proceed so I use my mobile in "Offline Mode" most of the time. Yesterday evening (driving back home) there was no sound again. So I played around a bit with the app. As soon as I disabled the OM the sound was there. I was able to turn on the OM after this again - for this drive. When driving this morning same again. I needed to disable the OM once and no problem.
So I think it would be a start to have a closer look to this when researching the issue - as far as I am not the only one experiencing this problem.
Edit another day later:
This info is only extending my last edit a bit. Today I realized that it is not only the Offline Mode as a feature of the app which makes the problem - I really need to have an internet connection at the beginning of playback. When I started this morning the OM was already disabled but there was no active internet connection and the problem was there again (title / track is shown on the display but no sound). So I was forced to pause and play again and again - as soon as there was a GSM connection the playback started.
So all in all I think the app does have a problem with a missing internet connection even though you've downloaded the audio tracks before.