1. The "Volume" button's text doesn't begin with a verb, as the HIG commands 2. Clicking on said button shows volume controls in a side panel, rather than a popup or something 3. It's kind of odd that the volume controls aren't visible by default in the first place; this is the more standard thing for video and audio players.
That is kind of by design. The user is expected to scroll wheel to change the volume as primary choice of interaction. That said. 1 Needs fixing and perhaps to remedy the problem a bit we could add a label to the volume toolbar that informs the user they can also scroll on the video widget to change volume - making it less of a hidden feature. Not having UI volume controls by defaults plays towards the minimalism goal of dragon.
I get that Dragon is intended to have an extremely minimal UI. I think I'd be okay with #3 being intentional if #1 and #2 were changed. Mentioning scrollability in the tooltip is a good idea as well. You reminded me that we need to do this for plasma-pa: https://invent.kde.org/plasma/plasma-pa/-/merge_requests/149