Volume rocker works as it should (at least on my device) - on default it changes ringer volume, during voice call it changes call volume, and when media is playing it changes media volume. Interesting thing is that when music is playing thru UAPP with USB DAC, and UAPP main screen is not in foreground, system still recognizes correctly that media is playing, and pressing volume rocker changes system media volume, not ringer or call (and when I pause playback in UAPP, rocker changes ringer volume, like in any other media app). On UAPP main screen things change - UAPP seems to 'steal' input from volume rocker and changes it's own internal volume (UAPP slider moves, actual audio loudness changes), but it doesn't change any of system volumes. What i meant was to set the listener (content observer ?) to monitor for media volume change, get android media volume value and set UAPP internal volume accordingly. There could be an option in settings like 'Link UAPP volume to Android media volume' for those, who want to retain current behaviour.
To put it in simple words: there should be an option to control UAPP volume setting not directly, but in response to system media volume changes ;).
To put it in simple words: there should be an option to control UAPP volume setting not directly, but in response to system media volume changes ;).