you can access the sounds volume by dragging of the Music reference and type Volume. Should be a Setter function for that.
As for using a slider, unreal has some built in slider widgets I believe, but I haven’t used them.
What the slider will do is output a 0-1 value though, and you can then use that to adjust your music volume. You may need to use a Map Ranged Clamped node to set the range of volume that you want 0-1 to cover.
Not sure why you pass this stuff back to the player controller though? It’s all stuff that is only associated with the UI menu isn’t it?