A super simple way of doing sound is to keep a “global” variable for your sound volume in your GameInstance. This would persist thru your gameplay and you could save/load it as needed. Whenever you need to play a sound you could just get that variable and plug it into the volume of your sound node and be done. You can even use a Blueprint Interface for your GameInstance so you dont even need to cast to the GameInstance to get the variable but just Get GameInstace->Get Volume Variable Function/Event.
This is not the best way but it is a good start and might get you thinking about other ways to get to what you want.
As for your slider part those are part of UMG and you can just use a slider to adjust your volume as needed. Conveniently they both go from 0-1 for your volume and your slider values.