I have an LG G6 running Oreo and whenever media is playing and a notification sound comes through something briefly lowers (not fully muted) my media volume. How can I stop this? I don't want notification sounds to change my media volume.
I first thought that the OP was asking about modifying the behavior of the sources of the notifications which can be done in with finer levels than before using Notification Categories in Android 8.0 Oreo.
However via the comments, it appears that the OP wants to stop Audio Ducking from happening in the media player. Unfortunately this is a feature of the media app playing the music itself and requires the developer of the app to configure. See similar question: Prevent music volume from being lowered when using turn-by-turn navigation
In Android 8.0 Oreo the App can now have Notification Categories
Check the notification setting screen in:
Settings->Apps & Notification->[App Name]->App Notifications
For the App(s) that are interrupting see if you can either disable sound or adjust the priority to one which suits your needs.
Rocket Player, is a music player that will let you choose between ducking and pausing, and will allow you to disable it completely, (The setting is under Settings->Sound->Audio Focus->Temp Focus Action)".
It does give the option to IGNORE all other apps. Just tested it: maps, camera shutter sound, notifications - all make sounds but no longer "duck" (more like stutter!) my music, listening via 3.5mm right now but should be same for BT.