This may require service repair if the previous trobuleshooting has not been successful. Before offering a ship-in repair, I'd recommend taking it to a UbreakIfix or visiting https://www.samsung.com/us/support/ubreakifix.
My phone does the same thing. It's quite a few people that are experiencing the same problem. What's happening is that your speakers are switching to "in-call volume." It happens on every single app that uses your speakers. Nothing that these people have suggested will help. I'm looking for a permanent solution myself. We may have to wait for a software update.
My phone does this as well and I have been waiting on a patch, but nothing. If a lot of people are having this issue, then it's a software issue for sure. Plain and simple. Hate when technicians say your phone is faulty when its widespread. $1000+ for a phone and I keep getting the run around.
Too many problems with Note9 speakers. Low notification volume, that this. And for monhts now. Nobody replied. Nobody fixed. Next time buy non-Samsung phone, their support is the worst at this moment.
$1000 for what? For issues? No thanks.