I resolved the same issue by restarting the Apple TV from the general on-screen menu. Hope it works for you!
From the main menu on Apple TV, go to settings, then general, then at the bottom of the list is the restart option. (Not reset)
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Will try and let’s see if fixes it. Thanks!
Here's what worked (for me)...
1. Have a friend connect to your Apple TV through 'More Devices' on the Spotify app. If they weren't connected before, they should have no issue starting to play music with sound.
2. Start playing a song on Spotify on your own phone and connect to your Apple TV through 'More Devices' on the Spotify app like before. The sound should start working perfectly.
Other unsuccessful things I tried before the method I described above:
-Unpairing my iPhone through the Apple TV remotes/devices menu options then reconnecting it
-Restarting Apple TV
-Updating Apple TV
This is the only thing that has worked for me, and it's worked over 5 times, without fail.
If anyone knows what this fix means on a device/networking level, *PLEASE* reply so I don't have to have someone come over and connect their Spotify to my Apple TV just to get the sound back.