I'd like to chime in and say that I think the option to have the volume slider horizontal rather than vertical, and to be able to choose where it is on the screen when being used, couldn't hurt and will only benefit users with different preferences.
For me personally, the vertical slider isn't as intuitive to use while driving and is difficult to access. And tapping the plus or minus over and over on the nav bar is a hassle from high to low volumes.
The option to have a Volume Knob pop up in place of the slider, after tapping the plus or minus on the nav bar, would be awesome as well.
More options for users, will only benefit everyone and having an even more customizable UI is only going to be an even better selling point for the Dudu7
Half the reviews I've seen for most Android Head Units mention people missing a volume knob.
Instead of making the volume controls a set configuration that is specific to Dudu7, give users the option to use a volume slider or volume knob and let them choose the location it pops up after activating the volume control from the nav bar.