So someone correct me if I am wrong. From what I understand is that you will need to make a sound class. If you are making a master volume control you will likely need to make a sound class for all sounds in game. Then make a sound mix for that sound class. Access that sound mix and then use the slider to effect its volume setting.
Here is a reference from where I am getting this info. Working with Audio in Unreal Engine | Unreal Engine 5.1 Documentation