I really like what you've done with this, the other volume widgets I've tried have been way to small or tried to do too much. I like the simplicity and size of your solution.
I have one suggestion though before jumping into adding the cool customisation features. It suffers from the same issue that all other volume widgets seem to have, and that is the volume jumps to where the touch is registered, meaning if your clumsy or accidentally touch near the max volume, you're likely to get a bit of a surprise.
I think it would be more useful if the volume knob didn't try to jump to the point where the touch is registered, and that it just becomes 'active' when you touch it. So the interaction would be more like touch the knob, nothing changes, but as soon as you touch it and rotate this is when the volume would change.
As it stands it's already the best volume widget I've tried, but if volume knob was a little more 'safe' for want of a better word, this would make the widget an absolute essential for all asteroid smarts.
As for customisation features, maybe the ability to theme the knob/buttons, or make the background more transparent. A source button might be nice for going straight to the active source or the source list.
Here is the update version that wont jump when you start the turn. In other words, if the delta for the angle starts from where you click. Let me know if it doesnt behave as expected, as I havent been able to test it on my unit