It's super simple to make your S9's volume buttons control media volume by default. (android.gadgethacks.com)
Where the notification adjustment menu used to pop up, you'll now see a menu for adjusting media volume instead. (android.gadgethacks.com)
Use this slider to increase the number of media volume steps — the lower you set the slider, the more volume steps there will be. (android.gadgethacks.com)