This is a software issue with Samsung....period.
I spent about an hour on the phone with Samsung explaining all this and advising them of two Note 9 speaker issue posts on their own forums, as well as many others online. A supervisor said she was escalating it and took down the post titles to have technicians take a look.
So I haven't had an issue since setting mine to mono and turning off Dolby Atmos.... That is I DIDN'T have a problem. My phone updated several days ago to 9.0 (a horrible version BTW), and the issue resurfaced....ergo software. Even a nontechnical person could figure that out.
I've worked in IT. I'm a certified network technician. I've taken phones apart, and rooted my old Samsung phones and tablets, so I'm capable of recognizing the difference between a hardware or software issue. And I know that its not related to third party apps when numerous people around the world are experiencing this issue and even having the same problems with brand new replacements.
I don't know if she really escalated it or not, but Samsung needs to stop jerking people around on this issue and admit there's a problem and work to fix it .
I'm not trying to be mean here, but there are now eight pages, on this forum alone, of people having this issue and there is still no fix or honest answer from Samsung. Just more suggestions for the paying customers to try, assuming it must be on their end.
I know the customer is always right doesn't mean a thing anymore, but businesses fail or survive based on their customer base, and Samsung is going to lose ground on this as they have started having more and more issues with their products.
I own three Samsung devices currently and have owned several in the past. I love Samsung products and have repeatedly recommended them. But their service has been slipping downhill.
I would hope it's only a software bug, but there is a very real possibility something is defective about either the speaker amplifier or whatever is controlling the equalization.
On Android Pie, it seems to occur less often, but sometimes the sound quality just sounds bad when starting an app and I have to restart it again and the bass sounds fuller. Dolby Atmos occasionally turns itself off as well.