So this is odd behavior. At first I thought it might initially just a specific app having an issue but since you referred to different ones it could be something relative to the operating system. And since you already tried a Factory Reset that negates trying to run your Nokia in its Safe Mode because of an issue with some user-installed utility app being a part of the problem.
Have you gone through everything sound-related in your phone's Settings menu? Perhaps there's some sub-menu buried deeper than the obvious.
If some overall sound setting glitch that's the source of this problem, this is a long shot but try booting your Nokia into its Recovery Mode and wipe the system cache partition.
http://www.hardreset.info/devices/nokia/nokia-8/
The system cache is completely different than the app caches stored in the user's data partition and it won't affect any of the data there so it doesn't require you to back up your data or anything like that. Note in Recovery Mode it's a text only interface so you need to use the indicated buttons to navigate through the various options, select the 'wipe cache partition' option, and it will only take a few seconds. Reboot your phone so it starts back up into its normal mode and see if the audio levels still drop down while playing.