You’re looking for Sound Classes.
But unfortunately the variables and functions you need are not exposed to Blueprints.
There are Sound Classes that already exist per default in UE4 and you can also create your own custom ones.
Then you can tell which class is a child of which one. Being a sound child class of another sound class means the volume ratio of the parent class is applied to all child classes.
Logically that means you’re going to have one Master Volume Class plugged to different child like Combat, FX, UI and whatnot, as you mention.
I just coded that yesterday in C++ for my A-RPG prototype. I want my prototype to be full Blueprint but I couldn’t find a way around that. I created a custom blueprint node accessible within my PlayerController. It’s far from being a clean solution but it’s good enough.
Here it is (don’t mind the Input functions, I had them on the same screenshots).
What I did: I created a custom PlayerController c++ class. Then I reparented my PlayerController class to that newly created C++ class. I then wrote down the few lines of code you can see in the screenshots and “voilà”. I have a custom blueprint node that allows me to dynamically change the volume of sound classes.
You can see the little dropdown menu on the node “change sound class volume”, when I expand it it gives me this:
These are all the sound classes available in the engine + the custom ones you created.
Here it is 
PS: I think that you can achieve kind of the same results with Blueprints only using the Sound Mix Class but I don’t know anything about how it works.