I made an account on this just to post this. For the last few months Spotify has seemed to automatically lower my volume when an artist would start singing or a particular instrument would cut in, and no answers I had seen would work, including changing the Communications settings to "Do Nothing."
If you have the same issue, here's what worked for me:
1. Type "Sound" in Taskbar, which should lead you to the Sound section of the Control Panel.
2. In Playback, right-click on your headphones and go to "Properties."
3. In "Enhancements, uncheck "Bass Boost" and "Loudness Equilization.
This worked perfectly for me, but I'm not sure if it will for others. Hope this helps!