I currently use a Galaxy S8 running Android 9 Pie and Bose QC35 II on latest firmware. I usually use the phone with its sounds muted, with the exception of the Media Volume when I'm playing media. About a day ago, it started playing notification alert sounds using the Media Volume setting instead of the Notification Volume setting. Additionally, it would play the notification sound multiple times overlapping depending on how many icons were currently in the notification bar.
I reset the settings of the phone which appeared to have solved the problem as notifications would only sound when the Notification Volume was unmuted. However, upon reconnecting my Bose QC35 II on latest firmware, the problem reappeared. I did a little bit of experimenting by clearing settings again and waiting a few hours with no appearance of the problem, only to have it once again reappear when re-pairing my Bose QC35 II headphones.
Even after disconnecting the headphones, the problem persists and notification sounds play multiple times overlapping on the phone's speaker as long as the media volume is turned up. The only way to prevent this is to blanket reset all phone settings. Seeing as I've had these headphones since January of this year and this phone since 2017, it's very odd that the problem has manifested in this way.
Additionally I have attempted to clear the pairing list of the Bose QC35 II as well as toggling settings in the Bose Connect app, which appeared to change nothing related to this problem.
I have noticed other forum posters with similar issues with other Android-based mobile devices. Hopefully these details will provide some answers for all of us.