if one is muted (in my case Spotify was showing the red block circle) you can unmute it by clicking the bottom speaker icon below the volume slider. You will see the Applications change based on registered applications.I have 2 distinct device profiles for my bluetooth headset, one mono, one stereo.
To confirm this, I had to go to a unique panel as follows: My symptoms were identical to the reported ones in the main post and replies: Spotify desktop app was not audible on only one particular audio output device, while all other app sounds and other outputs were working. I believe I've found the (a?) root cause: the spotify app was specifically muted on the problematic output device within the windows 10 settings.
