Here is another solution. This is how to add DAC volume control button on navigation bar.
1. Pull down quick settings panel and press gear icon for about 3 sec then release it.
2. "System UI Tuner" is added on the bottom of Settings>System. Tap "System UI Tuner."
3. Tap "Navigation bar."
4. Set each value as below, then close settings and you will see DAC volume control button on navigation bar.
(You can set "Layout" value as you like.)
*You can see a full list of keycodes (assignable key events) here. Each keycode is listed as "Constant Value" on the linked page.
Well done! Well found!