I was going crazy about this issue. I am a musician and almost an audiophile, and I could hear small fluctuations in the audio volume, almost in time with the drum beats, or with piano or any prominent instrument in a given mix. It was happening in any app, streaming, wired or wireless headphones, etc. Sounds like it is over-compressed.
Turns out it is the Dolby Atmos setting. I can switch it on and off and totally eliminate it! It is in Settings/Sound and vibration/Sound quality and effects and can be toggled off there.
Hope this helps!