The issue for the "pro" version is not resolved, since there is still no update available when this post was made.
Here is additional information:
Based on the app "precise volume" (where i adjust the volume to non-ear-bending levels),
one step (beginning from 0) via the hardware volume buttons is equal to 30% max. volume in the app (for media).
Everything else seems to be at 6% (100% / 15 volume steps -> seems correct), except for call volume, which is 0%
for some reason.
Since I do not use a SIM-Card yet, I could not test if there is a call sound. But at least for media there seems to be some
kind of additional multiplication happening.
Hope this helps a little in finding the issue.