I had a similar problem with my S9 and playing music via bluetooth speakers, one a B&W Formation Wedge and the other a Yamaha. The music randomly changed volume especially with bass and higher frequencies surrounding vocals.
I spent a week with B&W testing the speaker and also just spent another 2 weeks with Synology (music all stored on NAS drive) in case it was their music app causing the problem.
I saw a similar post to this and investigated adaptive sound and I saw that there are 4 modes on my S9, 1 = Off (no boost), 2 = Under 30 years old (boost high range frequencies), 3 = 30-60 years old (/boost high/mid range frequencies and 4 = over 60 years old (boost all frequencies). My phone had defaulted to mode 2 for some reason, so I switched to mode 1 Off (no boost) and it seems to clear the problem. I have tested a number of music files that were showing the volume fluctuations and are all problem free. I need to test my music for longer to be certain it is fixed permanently.
Using my wife's iPhone and airplay - no such issues.
My question to Samsung is why would you have the software set a default like this to an age range that is not relevant and importantly it should not be set at all and should default to off. I have spent a massive amount lof time investigating this - nightmare.
Samsung you are trying to be too clever!
